I recently purchased a tent and the handy tent set up guide had this recipe for eliminating mildew. Now I haven't tried it myself, but it is worth a try.
Mix 1/2 cup of Lysol in 1 gallon of HOT water. Sponge the mixture on the tent and let it dry. After it has dried, mix 1 cup of salt and 1 cup of concentrated lemon juice in 1 gallon of HOT water. Sponge the mixture on the tent and let it dry.
Store your tent in a cool, dry place. Storing tents in hot areas will cause the water repellent to deteriorate. There is nothing worse than finding out that your tent isn't water repellent, than on a camping trip in the pouring down rain... can you tell I'm speaking from experience here?