It only seems logic to apply some grease that is good for these helicals. You will find it on the net. And clean well before that, using an old tooth brush and some domestic petrol.
At one point I was talking to a Focomat owner about the difficulty of screwing the helicals into the lens stage and I liked his approach: he put the enlarger on the ground while watching tv and he just kept on trying very gently, going backwards and trying forward . . . sure enough at one point the thing would catch on and almost always without him realising at first . . .
If you don't manage I would try to find a repairer . . . better than forcing it !