Try
www.dishnetwork.com for starters.
The 501 has no DVR fees attached but you need to have it subscribed by Dish Network.
Does this unit have a smart card in it? I believe the slot for the card is in the back. Dish had a recent card swap. The new cards are yellow and the older ones were blue. At least in my case.
Hook the unit up to a tv. Look at the system info screen. It will show you the smart card # and the receiver #. Call 1-800-333-3474 and see if this box has a balance or not.