For live bearers like Platys, I'm pretty sure you use the usual 1 tablespoon per 5 US gallons. I would do this whether or not there is illness as live bearers like it slightly salty. Be sure to use aqaurium salt and not any other kind. You can buy the salt at your LFS.
I would also raise the temp of your tank to maybe 83-85 F or so for a little while, then lower it back down to 80 F or to whatver you have it on now.
You can buy meds like Melafix at your LFS and all you have to do is follow the directions on the back. I have had a good deal of luck with Melafix for various illnesses but you can go to the store and read the backs of all the different meds and guess which one might be right for you. I have found that its different strokes for different folks and that just because one thing workls for one person, it doesn't mean it will work for another. Do a little research at the store, purchase what YOU think will do the best and go with it. I wish I could give you the perfect solution but I have come to the determination that there isn't.
Good luck and keep me posted!