I thought that might be it because I had trouble figuring out how that worked too and the manual is not clear. The lever beside the threaded rod operates a half nut. In one position it engages the nut against the threaded rod so that the rod can adjust height up and down. In the disengaged position the router is able to plunge. This feature is different from other plunge routers and is also different than the M12V which I also have one of. I think the reason it is designed that , at least in my opinion, is because this router was specifically designed to be able to be used in a table. If you remove the black plastic base plate you'll see that 14mm nut on the end of the threaded rod and you can use it for above the table adjustment and it works well. Try moving that lever one side to the other (by the threaded rod that is) one way should lock it on the rod and the other way should leave it free to go up and down by pushing on it.
If you are having trouble locking it in position once adjusted that's a different problem and quite simple also.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.