Thanks for posting to our Forums! I understand that not being able to setup the Slingbox on your network can be difficult and frustrating. In this situation the first thing I would do is move the Slingbox directly to you main router and run through the setup of the Slingbox with out video. Then move your Slingbox back to where you would like to have it for the video signal. The most common cause for not being able to find a Slingbox to do the setup during a setup like yours is the Slingbox is not being able to get an IP address from the router, or the setup web site is having difficulties find the Slingbox on the network.
Here is an article that will assist you:
Hope this helps!
The Sling Media Moderation Team