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