First, you must find the specific IP address provided by the router for the Slingbox. Once you find this information, you must create the rule on the router. Then, finish the Setup Assistant for Internet Viewing.
The following link will allow you to get the step-by-step instructions in order to set up your Slingbox for Internet Viewing.
If you apply the right adjustments but the issue persists, this means that your modem is also working as a router. If this is your case, make sure to check this link.