Cablevision's most common mistake with a Silicondust HDHomeRun Prime and their CableCARD is not pairing it correctly. You won't know this until Media Center starts popping up with a message to call CV to activate service.
If your HDHomeRun Prime tuner status doesn't look like this, call them back at 718-617-3500, ask to speak with Advanced Services, and tell the CSR that you're getting a message about calling to activate service.
Thankfully, the folks in Advanced Services know what they're doing. *knock on wood*