It's impossible to tell from your image of the folder structure if it's a User path or a HD one.
The correct path to install MAC plugins is something like this - called the 'Root' Plugins folder:
**Macintosh HD**/Library/Application Support/Google SketchUp 8/SketchUp/Plugins
and NEVER this one....
*Users/A_User*/Library/Application Support/Google Sketchup 8/SketchUp/Plugins
even if that User is the 'Admin' - because then other Users - like you - can't see that folder !
I suspect you've put the stuff into the Admin Users path folder... and not the 'Root' one !