The instructions from the site are: here and below.

1. Look for your password on the bottom of your router or in the paperwork that came with your router. Copy-paste the following number into your browser web address:
10.0.0.138
Press enter.
Then Login with the password from the earlier part of this step.

2. Click the Firewall Settings button which will then be displayed on screen after logging in.

3. Use the Select a computer box to choose a computer to forward ports to. This box contains a list of computer names that are visible on your network. Only one computer connected to the router?

4. Click the Add a new user-defined application link.

5. Add Application name, example: WinMX.

6. Add port number in each of the 'from' and 'to' range boxes. Example, port 38120

7. Choose UDP checkbox.

8. Click Add Definition.

9. Click Back button.

10. Select the application name you just created in the Applications list (WinMx), then click the Add button to move them to the Hosted Applications box.

When you are finished, click the Done button at the bottom of your screen.