Let me answer this very tentatively, in complete ignorance of the equipment from DISH. If it is connecting using the coax connection, I am assuming it is decoding and routing the channels to your set. If this is using the old-fashioned Channel 3 or Channel 4 interconnection setup, then what you need to do is:
Hook the DISH converter up to your set, then do a channel scan. During the dialog with the channel scan, the set will ask you whether you need to have channel 3 or 4 available for external equipment connections. Answer this YES, and then proceed.
As I said, I am in complete ignorance of the actual equiment, but this is the non-obvious set of actions that might be blocking your progress. If I'm completely off-base, I apologize in advance.