I have been having like many the problem with the setup screen never locating my setup and no one from support responding to any posts what so ever.
So I have finally resolved this myself.
If you just run the setup as per the legacy Slingbox link then the new plug in is never installed on your browser. If this is not installed then you will find your slingbox is never found however long you wait.
So, go to your router and fix the IP of your slingbox.
Open port 5001 for both protocols and send it to the ip of your slingbox.
go to the setup page setup.slingbox.com and ignore the legacy link
Go the second link for the slingbox 350 and allow the browser to install the new plug in. this will involve shutting down your browser and for some reason slingbox seems to only like IE and not Chrome.
Once done Open Setup.Slingbox.com again and this time let the app run. For me it then instantly found my old Slingbox pro and set it up just fine.
Best of luck to all those who are stuck and being ignored.