What version of OS X are you using? I think that Xpress Pro 4.8 needs 10.2.x or lower. If you're running 10.4.x then you need the newest version of Xpress Pro (5.something).
I'm not at my office, so I can't share all of the information I have right now. Do some research on this subjuct and you will find a lot of information on it. Long story short, Avid did not update their software to be compatible with 10.4.8
You will need a patch, which I use or revert to an earlier version of OS X.
Like I said, do some research and you'll find what you need pretty easy.