Cats seem to delight in peeing in the most inconvenient places like on your mattress. The good news is that with a little patience and perseverance and some Odor Medicine O.F. Concentrate you can fix it.
Use your nose and a black light to locate the affected areas. Saturate each spot with a strong mixture of O.F. and work it into the mattress and let it soak for about 20 minutes. Then extract as much as you can with a wet/dry vacuum. Let it dry for about 2 days and then evaluate the results. If you still have odor repeat this process and you may try injecting the O.F. into the mattress with a turkey baster needle that you can purchase at a grocery store. It is very important to extract as much of the O.F. mixture as possible in order to ensure that it dries out completely in a couple of days. You may position a hair dryer to blow hot air on each spot to speed the process and help make sure it gets completely dry.