Thanks for reply. I've remapped all four control buttons (move left-right, jump and fire) to the letters Z, E, U, S. They showed correctly on the confirmation dialog and are working in-game. Can you actually enable Debug mode in your copy of Crystal Caves? Because I remember, that some command line option must be entered after the cc1.exe, to enable the Z+E+U+S cheat. I just can't find it on any of cheat-hosting sites.
I've just tested the cheats again and the ZEUS cheat works for me by default, but strangely, the XTRA cheat does not work for me at all. Only ZEUS and NEXT. I will look into this some more.
Edit: I have tested the game on my Mac, and all three codes work there. I'm not sure where the problem lies, with DOSBox, or with the keyboard.
I suspect the issue is with the keyboards. For instance, if I go to any text input field like on Google and hold down the keys x, t, r, and a in any combination, only 2 or 3 keys will register at all. I suspect the same thing will happen to you with the ZEUS keys. The only way to get the code to work may be to remap one of those letters to a different key for the game.