Final Cut Express is asking for my serial code

Around January time, I took my mac into the local Apple store because I was having problems with pixel anomalies. That was fixed and is no longer the problem, however, upon opening Final Cut Express for the first time the other night - the first time since getting it back from the Apple store - Final Cut Express is asking for my serial code which I no longer have as I threw the packaging away after installing it years ago.

What can I do to find my serial code?