What this guy said. I was thinking of getting a set of these molds myself. Definitely use 5000 psi high strength cement like jj did. If I remember right it only costs like a buck more a bag, and you know your stones are going to take a beating so it's kind of an easy decision.
My other idea, if you really want to go all out, would be to use some cement coloring http://www.homedepot.com/h_d1/N-5yc1vZ613l5/Ntk-All/Ntt-cement%2Bcolor/h_d2/Navigation?langId=-1&storeId=10051&Ntx=mode+matchpartialmax&catalogId=10053&omni=p_Less%20than%20$50
It's not necessary and is kind of expensive, but if you are going to make multiple stones with styrofoam or lead in them, it would be nice to color code them since you won't be able to tell the different weights apart without picking them up. Of course, spray paint would work fine too, but the cement stain will last longer and it looks really nice.