It seems that you are having some issues in order to configure your Slingbox for Internet Viewing since you cannot locate it on your network.
Most of the times, this issue is related to the security software running on your computer since it might recognize the Slingbox or the SlingPlayer as a "threat" to the network security. With this in mind, I recommend you to check this link:
I recommend you to reset the Slingbox. This process will allow you to locate it on your network. Once you finish this process, you will be able to get the IP address provided by the router in order to set up your Slingbox for Internet Viewing. The following links will provide you with further details and steps to follow:
The following third-party link will provide you with the step-by-step instructions in order to set up the Slingbox for Internet Viewing with the Airport Extreme: