Hi firedrake this might help u along all your multiple versions of su need to be 32bit only
and in profile i noticed your a mac user try d/l SketchyPhysics 3.7.3 version a lot bugs have been fixed
How to install:
Mac: /Library/Application Support/Google Sketchup #/Sketchup/plugins
(On a Mac, make sure that the contents of the SketchyPhysics folder you got out of the downloaded zip are in the Plugins folder itself, NOT in a SketchyPhysics folder within the Plugins folder.)
hope it helps
Tez