Make sure you download the most recent version from Red Giant. If that doesn't solve it let us know.
This is the version I downloaded: Red Giant - Products - Knoll Unmult 1.0, 1.1
If that's not the current version, let me know where I can get the current version
Also, I got it installed just fine on my Mac at home. My issue is on my PC at work.
If you're having trouble with Red Giant plug-ins, I recommend contacting Red Giant.