Locate where the smell is coming from. While this sounds like an unpleasant task, you will not be able to get rid of the smell until you find the dead rodent.
Remove the rodent while wearing rubber gloves and dispose of it properly. Wash your hands thoroughly after disposing of the carcass. If you cannot not find the rodent, try to determine where the smell is emanating from.
Clean the area where the carcass was found. This will help to mitigate the odor and will sanitize the area.
Set out a container of baking soda where the rodent carcass was found or the smell was located. Baking soda will soak up and capture bad odors from the air.
Find and purchase an odor eliminator and set it out near the source of the smell. Be sure you are purchasing a product that bills itself specifically as an odor eliminator, rather than just an air freshener.