I had the same problem were SlingPlayer or windows webapp would not see my slingbox classic when I moved it to another network where "I think it had an IP conflict" (even after resetting it to defaults). I had the SlingID from the SlingPlayer config and readded it manually using the SlingID info giving it a new name. If your slingbox is registered online the SlingID is listed there too.
If you are still having some difficulties finding the Slingbox on the local network, the issue might be caused by a security software on the computer that is blocking the application. You might want to verify that the computer is also connected to the same router.
I encourage you to perform the following troubleshooting steps in order to get this resolved,
I hope that helps!