^ Can't believe I didn't think of that. You're right. Although depending on what version he had, the code still wouldn't've worked anyway. =P
To be specific on this: The cheat codes as described by ccexplore will work on *all* versions of Lemmix, no problems. It's the normal level codes that, in some versions of the Original Lemmings lemmix player, the codes for levels 21-30 (in all ranks) don't work while the lower levels do; even if you have the correct Lemmix code (rather than a DOS code, which never has worked in Lemmix). This only affects Original; it does not affect any version of OhNo or H93 Lemmix players.
If you're using ccexplore's versions of Lemmix (the ones I linked before are ccexplore's versions with further modifications by myself (fixed the levelcode bug in Orig, and upgraded the H93 player to a H94 player), you can also press 5 during gameplay to skip a level. Contrary to the original games where you had to use the 5 on the numpad, in Lemmix you must use the normal number 5 (not the numpad). You also don't need to enter the cheat code first to enable this in any Lemmix player that supports it (except LPII and LPII Bonus).