I have had good success using fabric softener sheets placed under the seats.
Another option we used once in our office when the AC unit backed up was some kind of tub type product. I can't remember the name but we got them at Home Depot. We explained what happened and the person there told us about them. Basically you open them up and sit them on the carpet. It took about three days but you can't smell the mildew anymore.
Good Luck.