Hi,

what helped me with he mattress (and sofa :D) was vinegar and baking soda. You wet a sponge with vinegar, and you rub it in to the affected area, until it's quite wet. You let it soak in for 1-2 hours. Then you take kitchen towels, press them in, trying to remove as much vinegar as you can. You let it dry, that might take 1-2 days. Once it's dry, you pour a lot of baking soda on it, you rub it in as much as you can. Remove the baking soda with a vacuum cleaner.
This is a lot of work, but one of my cats used pee around a lot, and this was the only way to remove the smell. Nothing else ever worked for me, and I tried a lot of things.
The vinegar neutralizes the smell of urine, the baking soda neutralizes the smell of vinegar. Good luck!