Thanks Beccajo! That helped. I finally went and bought the cement they have at Michael's Craft (I'm impatient) and that worked, but I didn't get too much - only 8 lbs for $8 dollars. I'll try Home Despot again tomorrow and look for the smooth cement.
One more question: What should the consistency of the cement be when I start stamping? It seems that it is either too soft (and my letters mush right in there) or too hard and the cement cracks a bit around the edges of the letter. Is is just me, or is this a bit trickier than it looks?
I am in awe of you other crafsters - you make it look so easy!