I think the problem is that the new install of SlingPlayer- specifically, the Slingbox Directory- doesn't know where to find your Slingbox on the internet.
Slingboxes only show up automatically in the Directory if your computer is on the same local network as the Slingbox. Leave home, or use a fresh install away from home, and they disappear until you explicitly add an entry into the Directory using your box's SlingID.
If you know your SlingID, try Add a Slingbox... in your Player's directory and fill out the information.
If you don't know your SlingID, here's how to find it:
If you've done that already or it fails when you try, the SlingID service might have an out-of-date IP address for your home network. Try rebooting (power on/off, not hard reset) your Cable/DSL modem, router and Slingbox. This will restart everything, refresh your network connections, and prompt your Slingbox to update its location with the SlingID server.
Hope this helps,