It seems that you are having some issues with your Slingbox since it is not being recognized by your Airport Extreme.
First, we recommend you to check if the Slingbox shows solid lights. If not, check this link.
If it shows solid lights, you'll need to check the network connections (use a different ethernet cord, verify if you are using a power strip, etc)
If the issue persists, go ahead and reset the Slingbox. If you do not know how to, check this link.
Make sure to follow these recommendations and let us know how it works.