Simple Solution: its an enzyme cleaner.
You can purchase it at pet stores, pet smart, petco like places.
It may be in the doggy stain removal section.
The also carry it at places like meijer.
The pet stores carry it in a gallon container. YOu might want to get a gallon container for refilling, and a spray bottle for applying to smaller stains.
I've only used this on carpets, car seat, floor, clothing but not on a mattress.
What the goal will be is to saturate the mattress down as far as the cat urine went into the mattress. The idea behind the stuff is it eats the stuff away.
Then let it air dry. This will probably take a few days to see if it works.
You'll probably pour what you think is deep enough into the mattress.
If you put enough on the odor and stain will be gone.
If you didn't put enough on, then the odor will re-appear since not enough simple solution was used, or you may have missed some spots.
If you have an at home steam cleaner, or the portable hand size ones. You could try that as the cleaning solution w/ water. Since we have cats who have been sick, and are older. When we steam the carpets we use the simple solution stuff.
When i first grad from college, and was working a bunch of overtime, neglected the kitty boxes, one was peeing just outside the box. I tried for a month w/ every product to get rid of the smell. Didn't work, saw the stuff a friend from work mentioned. It got rid of the smell.
I had to pour the stuff on the carpet, enough to get into the padding, since I had used many other products and the urine smell would go away breifly with all the other product, but came back, that is until i used the simple solution.
It still took some work. I was using the stuff in a steam cleaner, and cleaning the area. But, on other days i was pouring in the spot. It was so long ago, I'm not sure how long it took, since I had spent a month trying a million other products to get rid of the smell.
I would also put some of the stuff in the laundry on the sheets, comforter it will help get the smell out of that stuff too.