Installed on my MacBook Pro Early 2011, and I've encountered this problem before where my mouse will move around the screen but won't select anything. I can't click to navigate anything and all has to be done through my keyboard. Just a little frustrating.
Update: I found the problem to be linked to my mad catz gaming mouse. When I have it plugged in, the cursor does as mentioned, but when I unplug it, everything's fine. But in-case this problem ever happens to me, an answer will still work for me.