Kelly, I don't understand your problem exactly so here are a couple of ways. If you have legs on the desk, bore a hole on the bottom of the legs for lag screws to fit there. If you need to adjust, screw the lag screws up or down.
Our hardware store has plastic shims that you can put under legs to level tables and desks. I think that they cost around $.25 each.
If you don't have legs in your design, run a level pencil line around the inside of the closet. It doesn't matter much where the line is because you can measure up and down from the level line to find other points around the desk. You draw the level line by marking along a spirit level. These are available pretty cheaply and are available down to a couple inches long but you want one that's about a foot long probably. You would probably like to use on that is as long as possible but one of the foot long ones leveled and marked around the walls is probably what you might have to settle for.
If none of this fits the problem, let us know more.