I got FCP 5.1 (from a friend...) but when I go to the System Update in my Finder, the update for FCP doesn't show up with the rest of the updates that are available. Why?

Have you updated you quicktime yet?
It is important to do so, because else it wont update the new fcp components.

Yes I think I have because when I go to System Update quicktime doesn't show up as something I need to update.

I'm presuming when you say you got it "from a friend" it's a dodgy version? Maybe Apple detects that and is prohibiting you from downloading the updates? That's the only thing I can think of...

have you tried getting the update direct from the apple site rather than through software update?

they must be a very good friend to give you their copy of final cut pro...

http://www.apple.com/support/downloads/