I've had trouble with installing Inquisitor myself, and happened to stumble upon a way to do it by accident. Thing is, it never stopped working on my older system, even though I kept updating it, so that I had Inquisitor running on 10.6.3 with Safari 4 and later 5. On the freshly installed system, you're right, a regular install doesn't work. Here's the workaround I used:
1) Get a system with a working Inquisitor install (Computer A)
2) Download
Inquisitor to the computer where you want to install Inquisitor (Computer B).
3) Copy folder /Library/InputManagers/Inquisitor from Computer A to Computer B
4) Run the standard Inquisitor install procedure on Computer B
I have no idea why this worked for me, because running the install changed the time stamp on the files in /Library/InputManagers/Inquisitor. Also, I can't guarantee that it'll work for everyone. I am interested in your experiences, though.