Mattress stains are actually pretty easy to get out. I had a water stain on mine awhile back, I simple took some clothing bleach to it. Take a cap full of bleach and pour it on the stain (more than one cap full depending on size). Make sure the area is covered then get an old toothbrush and scrub it.
Use a spray bottle, with a good strong nozzle to lift stain up, with 1 part bleach 3 parts hot water, spray area. Use a bissel little green turbo brush to scrub stain and remove the dirty water. Place the mattress in direct sunshine to dry. Sunshine will "bleach away" stains.
Apply a small amount of bicarbonate of soda or baking powder to the stain. If it's not wet, wet it just enough to activate the powder. Leave it for about ten minutes, then rinse out thoroughly with cold water. Repeat the process as many times as required until the stain has disappeared.