@tig said:

To find your correct Plugins folder for any SketchUp version... open that version of SketchUp and open the Ruby Console, then type in:
Sketchup.find_support_file('Plugins')
which gives you the full path to the Plugins folder...
Check for FULL permissions to that Plugins folder and if the expected plugin files and subfolders are indeed inside it

Each SketchUp version installation should be completely separate.
Each will have its own Plugins folder etc.
This sounds like a real mess...
If you do not use v7 then can I suggest that you uninstall it, and then uninstall v8 and reinstall v8 properly, or get a new install of v2013 - get new versions of your various plugins - many were rewritten to suit v1013...
Any existing Plugins folder and its contents [or other customized folders] should be unaffected by the system 'uninstall'.

Thanks for explaining what you rightly described as 'a real mess'!

Today I uninstalled TT_Lib2 folder, TT_ArchitectTools, and other TT plugins but not the associated .rb files. I opened Sketchup and this time the window titled "TT_Lib2 not installed", which earlier used to be blank, now displayed that I should install the latest version "TT_Lib2-2.8.0.rbz" (earlier I had TT_Lib_v2.8.3.rbz) with a download link. I then deleted the .rb files also from the Plugins folder and installed the new Lib2, then all other deleted plugins. Now Sketchup opens fine and there are no error messages.

Thanks again for your patience and help to resolve the problem.

In the Windows Explorer window for the Plugins folder, no button like 'Compatibility Files...' appears.
In any case, I have reinstalled LibFredo6_v5.2a.rbz but the status of my problem remains same.

A new thought. I mentioned that I recently went from Windows 7x64 to Windows 8 Pro x64, but my Sketch up 8 free is continuing from Windows 7. Can it cause a problem? Should I go for Sketchup Make which is the current free version of Sketchup? But if I do that, will I need a complete reinstallation of all the plugins?

@tig said:

You haven't entered details like OS in your user profile - I'm guessing it's a PC and v8 or newer...
The extensions installer can be fooled, if you have some restricted security permissions to the Plugins folder...
It might look like they are installed but some files/subfolders end up in a VirtualStore.
Windows protects you from yourself by limiting rights to access all folders within the Program Files folder-tree, by default...
To be able to add and manage plugins you need FULL permission.
To fix this you need to be an admin [if you are not one give yourself the power from the Users Control Panel]
Firstly close SketchUp.
Next... if you have given your Plugins folder a custom icon then remove it - doing this can slightly affect the folders permissions and although they report as 'FULL' they can be sufficiently restricted to affect installations...
Next...
Find the main Plugins folder and select its icon
Right-click > Context-menu
Properties
Security
Edit...
Set every user/group in the list to have FULL read/right permissions.
Apply/OK
Next...
Find the SketchUp.exe file in the same folder that contains the Plugins folder and select its icon
Right-click > Context-menu
Properties
Compatibility
Tick the box near the bottom, 'Always Run As Administrator'
Apply/OK
...
Now restart SketchUp and try reinstalling the tools...

Thanks for your detailed instructions.

My laptop is DELL Inspiron N5110, recently upgraded from Windows 7x64 to Windows 8 Pro x64.

After following your instructions, I reinstalled TT_Lib_v2.8.3.rbz and restarted Sketchup. First a blank window appears with title TT_Lib2 not installed, then replaced by a window titled Load Errors displaying the same error messages mentioned in my earlier mail.

A TT_Lib2 folder is very much there inside the Plugins folder (C:\Program Files (x86)\Google\Google SketchUp 8\Plugins), hence I do not know why it cannot be found by the application.

Anything else to try?

Recently installed TT_ArchitectTools and TT-Lib2 through the preferences>extensions route. When I open Sketchup 8, I get the following error message:

Error Loading File C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/bezier.rb
no such file to load -- C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/libraries/win32/TT_Geom_BezierError Loading File C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/gl_image.rb
no such file to load -- C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/libraries/win32/TT_Geom_BezierError Loading File C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/bezier.rb
no such file to load -- C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/libraries/win32/TT_Geom_BezierError Loading File C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/gl_image.rb
no such file to load -- C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/libraries/win32/TT_Geom_BezierError Loading File C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_UV_Toolkit/loader.rb
no such file to load -- C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/libraries/win32/TT_Geom_BezierError Loading File tt_uv_toolkit_2.rb
no such file to load -- C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/libraries/win32/TT_Geom_Bezier

My comments:
C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/bezier.rb - this is a valid path for bezier.rb file which is available
C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_Lib2/gl_image.rb - this is a valid path for gl_image.rb file which is available
C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/TT_UV_Toolkit/loader.rb - this is a valid path for loader.rb file which is available
C:/Program Files (x86)/Google/Google SketchUp 8/Plugins/tt_uv_toolkit_2.rb - this is a valid path for tt_uv_toolkit_2.rb file which is available
C:/Program Files (x86)/Google/Google SketchUp8/Plugins/TT_Lib2/libraries/win32/TT_Geom_Bezier - TT_Geom_Bezier is not available in that path

The drop-down list from Draw menu lists Bezier Curves. The Plugins menu drop down list shows Architect Tools.

My system is Windows 8 Pro (64 bit) and I use Sketchup 8 free version.

How do I eliminate the message?

I am keeping all plugins in the 'plugins' folder and not in any other folder of my choice. So deleted 000_AdditionalPluginFolders as you advised. The error messages are gone! Thanks.

On starting Sketchup 8 (free), popup window with following error messages appear:

Message from 000_AddifionalPluginFolders
The following plugins had errors and may not be fully loaded:
PLUGIN: win32api.so
FOLDER: C:/Program Files (x86)/Google/Google SketchUp 8/Plugins
ERROR: 127: The specified procedure could not be found. -
Init_win32api

The popup window also includes same error message for the following three plugins:

Raylectron
Raylectron Texture Exporter
Raylectron SketchyMesh

The messages started appearing after installation of said plugins. On clicking OK, popup disappears and other functions of Sketchup work normal.