I don't know how long you have to get this sort of complaint fixed. Usually there is a time limit for them to come back and take care of set up problems. Have you been complaining all along and do you have documentation to that effect? If so, I would make them honor those terms.

Now, as to the problem if it's too late to get them to fix it. I'm guessing what they would do is pull up the carpet in that area and take a belt sander to the high side of the plywood subfloor to level it off and relay the carpet. This might be something you could do or you could pull the carpet up, do the sanding and call a carpet layer in to re-install the carpet.