First, you need to know the IP addresses of your PC and your PS2. They should be static IPs, ie. they never change. The PS2 one you should know because you have to specify this yourself when setting it up. The PC's one can be found by Start -> Run'ing winipcfg.
Now, browse to your Router's IP address (typically 192.168.7.1). Click on Services, then NAT, then choose NAT Rules from the dropdown box. Now, click Add Rule. Here's the info you need:
Rule ID 2
Local IP address from & to: your PC's address.
Global IP address from & to: your global IP Address (visible on the router's home page; does NOT begin 192.168...)
Then, add another rule
Rule ID 3
Local IP address from & to: your PS2's address
Global IP address from & to: your global IP.
If these two both worked, delete the pre-existing Rule 1, then save and commit and reboot.