Assuming it's a valid number it's odd that it won't work. Some academic software will 'self destruct' past a certain date so you might trying rolling the date on your PMG5 back 10yrs. The activation servers might also have been taken offline. If it was FCP 5.0 (as opposed to 5.1) it won't work on your MB Air anyway because 5.0 and older required PPC CPUs to run (so either IBM or Motorola CPUs, not Intel). If it's 5.1 it might work, but it also might not because the hardware and OS is too new. For example, some older versions of FCP wouldn't launch if they didn't detect a GPU in an AGP slot, but over time AGP was phased out in favor of PCIe so people had to hack FCP to get it to work.