N.M.
You might try borrowing a carpet cleaning machine with an upholstery attachment. I've got one that I've used on the car seats and sofa. I think it might work on a mattress too.
And once you get it cleaned, be sure to get a waterproof mattress pad. We got one for every bed in the house (even ours and the guest bed, just in case they were there when it happened). I like the ones at Linens and Things--they are cotton-y on top, don't sound crinkly from the plastic, and fit our mattresses well.