Based on the information you gave us, that is a valid answer. Silly, isn't it?
So, did you do this?
Setting up firewire or ethernet
To Panther and Tiger, Ethernet and FireWire are almost the same thing; it?s easy to set up either.
1. Open System Preferences and click on the Network icon.
2. Choose New Location from the Location pop-up menu.
3. Name the new location Ad Hoc (for reference) and click on OK.
4. Click on Apply Now. (This will disable any active network connection you have.)
OS X will create a new location with an entry for each networking interface installed on your machine. If you view the items listed in the Show pop-up menu, you should see AirPort (if you have an AirPort card), Built-in Ethernet, and Built-in FireWire. The default configurations for each should work.
For FireWire, you now daisy-chain your computers, using FireWire cables. For Ethernet, you now connect two computers with a cable, or connect several computers with one or more Ethernet hubs or switches.
I have tried to set up a network between my iMac 2.4GHZ Intel Core 2 Duo with OS10.6.2 and my Mac G4 Power PC Processor 867 with OS 10.5.8. Why can I not get it to work?