Behind a Firewall/ AIM file transfer

If anyone is familiar with AOL Instant Messenger...
I've been trying to send files thru the file transfer section, but every time my friends and I try to make a connection, it gives a message stating that I am behind a Firewall. I have checked my Network settings and the firewall wasn't even enabled. I'm thinking that there is a firewall in LINKSYS. If there is, how do I change the settings or disable it?