Why can't I send/receive files in iChat?
This link is old but may help, 
http://www.macobserver.com/tip/2007/02/14.1.shtml

I also noticed that it might help to just drag and drop the file you wish to send over the person you wish to send it to in the drop down chat menu and drop it there instead of on the chat box, it will then ask you if you want to send, click yes and it should work.