GI_Joe works fine on OSX. Just make sure you follow the directions exactly. You need to change line 57 in GI_Joe.mel to point to GIJoe.lights in your scripts directory. The line should look something like this:
string $pathL = "/Users/danny/Library/Preferences/AliasWavefront/maya/scripts/GIJoe.lights";
I used BBEdit to modify the mel script since it has line numbering built in. However, any text editor should work just fine.
The rest of the directions works just fine. Also, if you want to use the shelf button icon, you need to convert it to an XPM file using GraphicConverter. Maya for OSX does not recognize other image formats for shelf buttons. To save you some time, I've attached it to this post.