OK: finally got the beast to work.

Manually placing the files into the plugin folder just won't work (even though several instruction files tell you that this is possible). Almost certainly a waste of your time.

The only way I could get it to work was to install the fredo lib file in sketchup via:

window/preferences/extension/install extension

Fredo lib:
http://rhin.crai.archi.fr/rld/plugin_details.php?id=605 .... CONTENT DELETED (SEE TIG's post below)

CORRECTED METHOD as per TIG's advice below:

1 install the SCF Plugin Store tool
http://sketchucation.com/resources/plugin-store-download

2 via the store plugin, in sketchup, install the latest FREDO LIB.

3 then choose 'roundcorner' (or any other desired plugin); install from there.

The process is actually very simple, what fouled me was existing install how-to instructions that advised manually placing the files in the plug-in folder, which just doesn't seem to work with this version.

Thanks TIG,

I've edited my previous post, as I don't want my previous inaccurate info to stand, as there's more than enough of such out there already!

I updated the lib from the store, no problem. FWIW, the old lib worked fine, but no reason to take any chances. I wasn't doing anything extravagant with it, just trying it out on some primitives.

Yes, some of those old 'how to install' links should be fixed, so.

The number of people who had the same problem as I did is extremely high - he must be losing a phenomenal number of users as a result. Which is a shame, because it's a fantastic tool.

Anyway, thanks again.

Just an update: i've continued to struggle with this, carefully following Fred's install guide, but nothing works.

I've downloaded 'round corner' and the library file; I've installed them as per the instructions in the install pdf, and I still get this:

Error Loading File LibFredo6.rb
Error: #<ArgumentError: invalid byte sequence in UTF-8>
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LIBFREDO6_Dir_37/Lib6Core_37.rb:369:in match' C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LIBFREDO6_Dir_37/Lib6Core_37.rb:369:in block (2 levels) in load_from_config'
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LIBFREDO6_Dir_37/Lib6Core_37.rb:368:in each' C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LIBFREDO6_Dir_37/Lib6Core_37.rb:368:in block in load_from_config'
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LIBFREDO6_Dir_37/Lib6Core_37.rb:367:in foreach' C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LIBFREDO6_Dir_37/Lib6Core_37.rb:367:in load_from_config'
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LibFredo6.rb:60:in register_plugin' C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LibFredo6.rb:52:in block in startup'
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LibFredo6.rb:34:in each' C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LibFredo6.rb:34:in startup'
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LibFredo6.rb:71:in <module:LibFredo6>' C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/LibFredo6.rb:29:in <top (required)>'
Error Loading File ZLoader__RoundCorner.rb
Error: C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/ZLoader__RoundCorner.rb:32: invalid multibyte char (UTF-8)
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/ZLoader__RoundCorner.rb:32: syntax error, unexpected tIDENTIFIER, expecting keyword_end
text = "Problème avec l'installation du plugin #{fro6__plugin}"

		                ^

C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/ZLoader__RoundCorner.rb:33: syntax error, unexpected tIDENTIFIER, expecting keyword_end
text += "\nLibFredo6 n'est pas install\é ou bien n'est ...
^
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/ZLoader__RoundCorner.rb:33: syntax error, unexpected $undefined
..."\nLibFredo6 n'est pas install\é ou bien n'est pas dans le b...
... ^
C:/Users/doc/AppData/Roaming/SketchUp/SketchUp 2016/SketchUp/Plugins/ZLoader__RoundCorner.rb:33: invalid multibyte char (UTF-8)

Beating my head off the wall all morning. I have the same problem, but the fix here as described (or as I'm trying it) doesn't work. Any ideas? Thanks.