Well.. you left alot of holes in your question, but I'll try to answer the best I can :]
First of all, are you networking the two computers together directly? Normally you would need a special networking cable called a crossover cable to directly network two computers together, but it really shouldn't matter what kind of cable you're using to do this because the Titaniums have auto-sensing Ethernet ports on them so they'll work using standard patch networking cable.
I'm also assuming the iMac is running OS X too? If so, just make sure both computers Network preferences have the "Ethernet built-in" network port is active. If they are, make sure under the AppleTalk tab that AppleTalk is active.
From this point, go to the Sharing preferences on each and make sure File Sharing is turned on for both computers. You should be able to see each other on the network now, but if not, try restarting the computer after turning File Sharing on.
Hope this helps.. try to be a little more specific with your questions : ]