Before you do anything else, lay the futon out in the sun, mildew side up--when it is good and dry, the mildew will be dry, too--then get a really good vacuum and clean it. If you feel inclined, instead of Oxyclean, buy a box of Arm and Hammer washing soda and a box of 20 Muleteam Borax and mix them(this is what Oyclean is, but it costs about a quarter as much) and do whatever with it.

After all this,through the futon out, because even if this works, you'll never be able to get the mildew out of the filling;-)--