The original IdleKeys author started with some sample code to deal with the global hooks. Since I'm a .NET developer myself, I decided to take a peek and discovered where the exception was being thrown, and found an updated way of accomplishing the goal.
I've attached a new build of just Gma.UserActivityMonitor.dll. Just drop that file into your IdleKeys folder (with IdleKeys not running, of course), and it should magically work for you.
Note that I compiled against .NET 3.5.1. So you'll need that, or later, installed, which I'm pretty sure everybody will.
If a forum moderator wants to repackage the original Zip with this new DLL, feel free. Or I can. I just don't want to take credit (or blame!) for the original application. :-)
Brad. Gma.UserActivityMonitor_Fixed.zip