Make sure you're in the same folder as LemmixPlayerExtra. Then look for the INI file; that is the configuration settings. You will see a list of settings. The first one, CheatCodesEnabled, may have a 0 next to it. Change that into a 1.
After that, go in the game, and press F2. You will see a password screen. Then you can either type the rank name and level, or rank number and level.
If you want to go to Amiga 2P level 21, for example, you would type in the password screen:
AMIGA2P21
or
0721
This will work in all players, traditional and Neo.