I'm not ruby programmer but some installers require you a specific directory. May be the loader expects to find some things on a specific placement.
As you know we can get several user accounts with the same directory structure and SU will search for plugins in a determinate order (I read it somewhere). I'm not sure but it could be in the plugins folder inside SU application, then the general library, shared library, user library, and so on.
I just collected all of them in my user account/library/application support/GoogleSketchup 7/Sketchup/plugins. All excepting those that are using an installer but probably most of them would work the same.
Finally, I don't understand why some rubys needs another location to work.
Some problems may need file permissions verification (I tried all possibilities with pipping plugin without success.