It's a known issue. Myself and others have posted about when it will be fixed with no response from Bungie. Bungie has already released 2 patches which don't include a simple fix for this problem. It was working in the beta a month ago, and now it doesn't. You can't map the ENTER key or the NUMPAD keys. Extremely frustrating, but doesn't seem to be a priority for Bungie, as it should be, considering this involves the input device used to play the game.