I understand that you are having some issues in order to set up your Slingbox PRO HD with the ASUS RT56U router.
Although you have provided some information, we need further details. Is the router recognizing the Slingbox? Does the Slingbox show solid or blinking lights? Are you using a direct connection between the Slingbox and the router?...etc.
We recommend you to provide further details about your network layout and error messages (if you get any), this way the community will be able to provide you with accurate responses.