Assuming my last post was correct and your old Mac IS running OS X, you will also need to enable Personal File Sharing. Go System Preferences > Sharing > Services and click Start after highlighting Personal File Sharing. Make sure you give your old Mac a name in the box above.
I would only bother turing on Sharing on the old Mac. Its name should show up in the Network portion of the Finder window on the new iMac. If you click the name, it should then show an icon with a Connect button underneath it (assuming your finder window is in column view).