I've not flown it myself, but your most likely options are to fly to FRA on UA or LH and connect on LH to CMN. Or, connect in Zurich and take Swiss to CMN. (I have not actually checked the route maps to see if LH or LX flies to CMN, but I'm guessing they do).

Edited: LH definitely serves CMN out of FRA and LX/Edelweiss Air serves it out of ZRH (though not daily). LX serves Marrakesh too.