Just added this to my mac and works like a charm!!
Hmm... I've got a ton of plugins, so showing the folder structure wouldn't really help but...
I installed it like so:
/library/application suppor/google sketchup 7/sketchup/plugins/
- Libfredo6_Dir_34 (folder)
- Libredo6.rb
- ROUNDCORNER_Dir_21 (folder)
- ZLoader__RoundCorner.rb
(Note: I had to delete an older Libfredo6_Dir. I also had to delete and older ROUNDCORNER_Dir, but it worked fine with this older directory in place)
What a great thing... awesome.