You are probably running under NAT 3, which basically means you are behind a firewall and a router. To get yourself into NAT 2 (which will open things up) you will need to open specific ports on the router for the PS3. This will also require setting up a specific IP address for the PS3 in the router.
If you aren't sure how to do that let me know what brand and model of router you have and I can look up the instructions on how to do it.