The issue might be that sharing isn't enabled for both of the computers.

This could take me a while to type everything out, so I found a link to help navigate this a little easier. Try http://www.ehow.com/how_5030012_network-mac-pc.html This should get you setup to share items from one to the other. If that doesn't work, you might have to create an Ad Hoc network to get them to talk.