One thing to do is clean the mattress with a bacterial enzyme odor eliminator product such as Nature's Miracle--very, very deeply. Then if there are any further accidents, pour undiluted clear vinegar deeply in. If you want to sleep on the bed before that dries, put a bunch of folded terry cloth towels over it and night-night! But leave it open to the air for awhile other than your sleeping times.
Vinegar seems to repel dogs from wanting to use that spot again. No comment to the dog when you apply this, but you can let the dog watch. Hopefully your trainer has taught you that punishment doesn't work for things like this. It just doesn't!
I would suggest crating the dog at night. How about start by doing that for two weeks, and then try on the bed again?
You can get lots of details on Housetraining at the link with my signature, titles starting with the word Housetraining, and they are alphabetical. There is no fee and no need for registration, either. The site is provided to the public by the Veteriary Information Network.
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47