There have been a few folks who had something apparently wrong with their height adjustment, and that may be the case here-- the MMM height adjuster is just a threaded rod, and there's a nut/washer below that; it's supposed to limit the downward travel of the two MMM lift arms (but not the 2 3PH lift arms at the rear of the tractor).
The idea is, as Axle mentioned, to set the height adjuster (the knob on the floor with "+" and "-" on it) all the way up, in the highest position. That should allow the 3PH arms to travel normally, but mechanically hold the two MMM lift arms in their uppermost position. As I said, some people have had situations where that didn't work, however -- something wasn't right. I don't recall what the solution was, but I believe they ended up going back to the dealer.