I'm sorry if this have been brought up before but I can't find any information on which passwords are being used. Are there any possibility to change to an desired level within, for example, the genesis levels? If I have played the first 19 levels and want to start at 20, are there any possibility to do so?
The passwords used in the Lemmix players are unique to Lemmix (they're also different between NeoLemmix and traditional Lemmix; and are different for each game).
Yes, there is a way to choose a specific level. First you must activate cheat mode by entering the correct code on the password screen. In all the traditional Lemmix versions of official games (including Extra and CustLemmix), the code is "CHEATCODES"; while in NeoLemmix, it's whatever it was in the official game.
Once you've done this, you can choose a level by entering the rank name and level number as if it were a password, eg "TRICKY21". Additionally, when cheat mode is active, you can skip a level by pressing 5 while playing it. You don't have to reactivate cheat mode each time you launch the player; if you activate it once, the player will remember that it's been activated every time you run it. (By the way, if you ever want to turn it off for any reason, just enter the activation code for it again.)
Official game (and NeoLemmix) codes:Original, Prima, Covox - BILLANDTED
OhNo - SLAMRACING
Xmas91/92 - SNOWFLAKES
H94 - ENTERPRISE
Extra - CHEATCODES
CustLemm - SKIPLEVELS
* The H94 code doesn't work on the DOS version, but I believe it does on some other versions such as Amiga
Lemmings Plus series codes:LPDOS - NIGERFAGOT
LPII - NGGYUNGLYD
LPII Bonus - BARRELROLL
LPIII - HEARTBLEED
LPIII Bonus - FISHSTICKS
HLP - MYSTERYBOX
* Note that cheat codes can't be used to access the secret levels in the standard versions of LPII and LPII Bonus (but they can in LPIII and LPIII Bonus, and the NeoLemmix versions of LPII and LPII Bonus)