Still, mildew is a very bad thing when it comes to tents. It smells, for one thing. Its little black spots are unsightly. Worst of all, perhaps, it weakens the material. But, there is a way to kill the mildew spores, get ride of the smell, and do a pretty good job of removing the stains. Here's an oft-cited anti-mildew recipe:
First, kill the mold with a solution made from one-half cup Lysol mixed with a gallon of hot water. Rinse with a solution that consists of one cup of lemon juice and one cup of salt, also mixed in a gallon of hot water. Allow to dry, then soak the stains in another batch of lemon/salt/water solution. That should help remove them. Rinse thoroughly and allow to dry. Do NOT put tent in clothes dryer, as the heat will cause the waterproof coating to peel off, and then you have real problems!
Lastly, store the dry tent in a breathable, loose cotton bag to prevent future mildew outbreaks.