First, you need to gather the specific IP addres provided by the router for the Slingbox. Once you get this information, you need to access your router interfase in order to create the rule according to the Slingbox IP address. Finally, you will use the same IP address in order to finish the Setup Assistant for Internet Viewing.
This link will allow you to get the step-by-step instructions in order to set up your Slingbox for Internet Viewing.
Since you did not include the specific model for your Belkin router, the following third-party links will provide you with the steps to follow.