Originally Posted by theAdmiral
This does not open files in a program dialog. It only opens files in an explorer window (to my knowledge).
Yes, you are right. It will also work on your desktop (Nautilus), but not really with other dialogs.
As for the error message, "Mousetweaks need assistive technologies" - you should navigate to System > Preferences > Personal > Assistive Technologies and click on the "Enable assistive technologies" checkbox. Click the "Close and Log Out" button. When you log back in, assistive technologies should be enabled.
I tried the "Trigger secondary click by holding down primary button" option, but the "secondary click" seems to be a right-click.
You probably want a double-click. Unfortunately, I'm not sure how to do that.
Hopefully someone else has a better idea.