You'll need your local IP address's last 3 numbers. If you don't know them, do this: [NOTE: This is for Windows XP, but it should work in Vista]
Right click on the wireless icon in the system tray and click on status. Click on the support tab and it will tell you your ip address. You only need the last three numbers. Keep them handy.
Open a new tab or window in your browser and type in 192.168.1.1. It will probably ask your for a username and password. By default, both are "admin" (no quotes). Next, click on "Applications and Gaming." You'll be on the port rage forward page. Fill a single row of the table like this:
SRB2 | 5049 | 5049 | UDP | XXX | [checked]
After that, click save settings and you should be done.