I bought Aperture already when I own my first MCP
Did you buy a boxed retail version? Then the serial number is in the box on the leaflet. Or if you downloaded the trial version and purchased a key, you will find the serial number in the email.
But if Aperture is still running on your old machine, you can see the registration code in the "About" panel. From Aperture's main menu bar: Aperture > About Aperture.
The newer Aperture versions from the App Store do not have a serial number and should not ask for it.
Regards
Léonie
If you cannot find the box with the serial number and have no longer a running Aperture installation contact Apple for a replacement: Pro Application Replacement Serial Numbers