First some general points - I see you are on Vista - are you the 'administrator' ? If not then when you put files into the Plugins folder they might seem to be there... but to protect you from yourself Vista silently puts them into a 'compatibility folder' off to one side - look to see if that's the case.
If they aren't in the Plugins folder they won't 'load' when Sketchup starts. Ensure [as an 'administrator'] that your ..Sketchup.../Plugins folder has its properties set appropriately, so you can add/change files in it without such problems cropping up...
When you download files they must go into the correct folder[s]. A simple lone '.rb' script goes straight into 'Plugins'. Note that some script files are encrypted as '.rbs' to protect the author's code etc. All .rb/.rbs files within the Plugins folder auto-load when Sketchup starts. However, many tools are now so complex that they need a lot of supporting files, and to avoid cluttering up the Plugins folder with these many authors provide a .rb file that goes into Plugins [a short 'loader' script] and then many more files in their own sub-folder[s] within Plugins that that 'loader' uses. All of a tool's files usually come compressed up into one file - a 'zip' - but rarely in a 'rar'; keeping the files' sub-folder[s] structure - so you must 'extract' everything from the 'zip' into your Plugins folder, keeping all of the files within their sub-folder[s] - otherwise the loader won't find what it needs and fails. Don't be tempted to drag and drop files from a zip window as this can loose their sub-folder arrangements - use your zip program's 'extract' option and most good zip-apps keep the sub-folders by default...
A few tools need additional supporting files that aren't supplied within the zipped set, but then that tool's download page usually explains this and has links to get these additional files. For example Fredo's tools usually need his 'Lib' language translation tool bits too, and some tools use 'helpers' like 'progressbar.rb'...
Do you get any error messages at startup?
Also open menu 'Window' > 'Ruby Console' to see if there are any error messages there too...
Tools usually have clear instructions on their installation on their download page.
They often have usage 'instructions' on that page too - or sometimes as a link to a pdf etc. There are also instructions inside many scripts, written at the start - you can actually open a .rb script with Notepad.exe'' and simply read it - a script is actually a plain-text file. If it's not clear from other instructions, then the menu setup usually comes ate the end of the script's text, and that should also indicate the menu name etc where the tool becomes available when loaded...
So, all .rb/.rbs file within the Plugins folder auto-load when Sketchup starts. If they can't find any 'required' files then error messages appear then...
Some rubies are so rarely used that they are [re]named with a '.txt' suffix so they don't even auto-load - but you can load it manually into a session for that model by typing into the Ruby Console - load "rubyname.rb.txt" - it then becomes available to you for that model's session only...
Note that some scripts that do auto-load appear to do nothing - or at least nothing you can actually 'see' - like the 'progressbar.rb' which as a 'helper' script can add an extra function to other tools.
Some tools are so rarely used that they aren't given menu items etc, and are only activated by typing their command into the 'Ruby Console' - e.g. my 'cpointseqxy.rb' tool - where you type in cpointseqxy: the download pages explain each tool's usage.
Most tools load and get new item appear in the 'Plugins' Menu [the normal location], or perhaps sub-menus within it - e.g. my 'Extrusion Tools'.
Some tools are set to appear in other menus - e.g. 'Joint Push Pull' is found in the 'Tools' Menu, with its own sub-menus - see its help PDF file. Othe tools appear in File/Draw/View etc depending on their functionality. Their help-files / instructions almost always explain where to find them...
Some tools appear in the right-click context-menu - e.g. 'Joint Push Pull' which also appears there as well as under the 'Tools' menu - or for other tools, like my 'SectionCutFace', it is the only place they appear, and they only shows when you have the correct type of thing[s] have been selected - in that tool's case you need to have one just 'section-plane' selected to see the option at all.
Some tools have their own 'Toolbar' [either as their only interface or additional to their menu entry] - to activate a Toolbar see the 'View' Menu > 'Toolbars' list and tick the one[s] you want to be active. Place the loaded toolbar[s] around the screen's perimeter or floating as desired. Note that all toolbars need button image files etc and these usually go into a special sub-folder as explained above...
Additional to all of this, some tools also come to load as 'Extensions' - and then these usually start their life 'disabled', so you get no menu/toolbars showing at all. To 'activate' a tool that is an 'Extension' open the 'Window' > 'Preferences' menu and click the 'Extensions' tab - scroll the list and find the one[s] you want and tick to activate. This is a way to switch various Plugins Extensions on/off [although it does need a restart of Sketchup to become effective], without you having to [re]move the .rb files [or rename .rb files as .txt] within the Plugins folder. Note that an Extension's status is remembered across sessions - so if an Extension is activated today then it's still activated when you use Sketchup again tomorrow.
Hope this was not too daunting and helps...
Once you get the setup right you'll find Plugins are easy to install and every useful to have...
