Here is how you have shimejis on a Mac:
1. Download the desired shimeji file
2. Go here, where you download the basic Mac Shimeji.
3. Extract the files with an archiver, I recommend B1 archiver, in which you can download here.
4. Open both files with Finder.
5. Replace the "conf" and "img" files in the Mac Shimeji with the ones from your desired Shimeji's file.
6. Enjoy your little cuties!
I hope this helps!
HOW TO MAKE IT INTERACT WITH YOUR WINDOWS:
1. Open System Preferences
2. Go to Universal Access
3. Check off Enable [something something] (the first checky thingy???)