what is the active ingredient in automomatic transmission stop slip?

Snake oil is the active ingredient, you know what snake oil is right? stuff they sell you that makes you think it works but in reality is just a gimmick that does nothing but part you from your hard earned money, in the case of stop leak and anti slip its just an alkaloid based oil such as brake fluid that softens the seals and swells them up, this only temporarily works if your slip is due to an internal seal leak within the clutch packs, if its due to the friction disks being worn or other problems then it does nothing, unfortunately in your case being a quart low will not cause your trans to slip, you have other issues, and I seriously doubt snake oil will fix your problem, and yes I understand times are tough, unless you happen to be a mechanic which seem to be in high demand these days, but your going to have to bite the bullet and have your trans rebuilt or hit the wrecking yards for a used one, myself I drive a newer truck and have no problems, and even if I did I could repair it or just go buy a new truck.

It won't matter. When an automatic transmission starts to slip it's because the oil pump is failing and the clutch packs get fried. It won't move soon no matter what you do. A rebuilt transmission will be needed very soon. I would not even drive it, all you will do is get stuck and need a $100 or more tow.