Can't connect two macs via Ethernet cable

I just bought a new MacBook. I would like to use Migration Assistant to move everything from my old iMac G5 to my new MacBook. Since the new MacBooks don't support Firewire, I would like to transfer everything via an Ethernet connection between the two computers.

I plugged in the Ethernet cable into both computers but both computers don't seem to recognize that the cable is there. On the G5, I get the message, "Cable for built-in Ethernet is not plugged in", and on the MacBook, I get the message, "Cable unplugged".

Help! Thanks in advance.