Run xf86config or edit the file to 'emulate 3 buttons'. If that's okay, then sometimes some applications just don't play ball. This happens in every app? And last, it ain't easy to do for some reason. I went and bought another mouse because I was tired of pasting about 50/50. *g*
I got an IBM optical scrollwheel that I can click as a third button for 15 bucks. Very much worth it to me. 'Course, I tested it with another computer's scrollmouse first but I didn't want to deal with a rollerball, either.