Three things to try:
1) Make sure the port you're (both) using to send file through is forwarded to your computers. You would need to set your SMC to forward the port under My AIM>Preferences>Options>File Transfer and your friend would need to set his Netgear to do the same, using the port specified in his AIM client.
[edit]You can set a port under Options>File Transfer. Go there and look and/or set a number -- just about any number works, really. On your router, forward that same port number to your computer. If you don't know how, read the documentation that comes with your router. Get your friend to repeat that process.[/edit]
2) Check off "Use Alternate Internet Address" in the same area of Options.
3) Have each of your computers set as DMZ on your routers.
Sometimes the connection takes a few seconds to negotiate, so be patient.
~Ladi