Don't know how you missed it, but versions of FCP prior to 5.1 will NOT run properly on any IntelMac.
FCP versions 1, 2, 3, 4, 4.5 and 5 were written for the older PPC processor Macs. Starting with version 5.1, FCP was/is written for BOTH PPC and Intel processor types (universal version).
-DH