Yes, I am using Mountain Lion and it is fairly new to me. Even had trouble locating Library which they took out of finder for some reason!
I had no luck manually installing new folder, but found ThomThom's Simple Plugin Installer, and was successful using that and re-installing zip file using that. Thanks for your help.

You were correct in thinking the Plugins folder was set to read only, however even after changing that (to all users) the option for New Folder is not highlighted. I then went up the feeding chain to Sketchup, Google Sketchup 8, Application Support, Library, and Macintosh HD and changed those to read & write to see if that would make any difference. It did not.

This seems like an unusual amount of tinkering to get a plugin to work?! Is this common?

I am trying to install roundcorner plugin for Sketchup 8 on Mac running 10.8.2
When I start SU I get this message and am not able to fine toolbar or plugin:

Cannot load plugin LibFredo6 because the script cannot create DEFPARAM_Dir folder in:
/Library/Application Support/Google SketchUp 8/SketchUp/Plugins

I read in a post that I need to create the folder manually, but when I go to the plugins folder, the menu option is not available to make "New Folder" (shift+command+N does not do anything either)

How can I create this folder, or get this plugin to run??