Are you behind a router, or is your PC firewalled? Try enabling uPnP on your router if it's not already enabled. If that doesn't work, you may have to find out what ports MP uses and forward those to your PC, or add exception rules for them on your PC firewall.
I play on xbox, so I'm not 100% sure on the details of PC gaming. Isn't the PC version run on dedicated servers, though? Why is having moderate NAT an issue?