Okey dokey.
REally, read this thread:
Donkey Dick and VT local IBL Indoor/Outdoor 2008
I just found the same problem, that it is difficult to source quality soils at a reasonable price in the dead of a New England winter! So this is my first ProMix grow.
You don't have to add a thing to it, but you have to make certain that you don't leave a plant in the same sized pot for more than a month to avoid the pH problems that ProMix users sometimes bump into.
It's soilless, so you run your pH just slightly lower than for soil- 6.4 is about right.
NEVER overwater the stuff; that shortens its lifespan in the pot. You can flush it if you have to, but you really need to let it dry out a bit between waterings to get the benefit of its texture.
As you might see in my log, I found that the lowest set of leaves DID turn yellow and drop; how much of that was light and how much was pH I'm not sure; growth was good in general and I made sure that I was following good watering and feeding routines.
The sprout will be fine with no added nutrients until about 3 weeks (differs based on how much light you have), and then you just give it regular grow fertilizer like any other medium. The Advanced Nutrients line was designed to be used specifically with ProMix, as that is a popular choice for large-scale commercial growers.