Jason,
The idea with slack tenders is that, when you pull the tail of your rope, the pulley (or snap or carabiner) will push the bottom of the knot, advancing it and allowing you to strip out the slack with one hand. It doesn't really matter how you attach it, since it is not a load-bearing part of the system. You can tie it to the harness with a shoelace if you want.
I think most guys clip a biner or snap into the dee ring that they're tied into below the primary biner. You could also clip into the main biner itself, but you won't be able to remove as much slack from there. If you want to experiment without spending much money, get a utility biner or a keychain biner (a buck at the hardware store) and clip it to the main dee below your tie-in biner, then around the tail of your rope. A pulley will give you a little leverage and won't bite into the rope, making it easier to pull out slack, but a biner does the job and will let you get used to the principle.
Advanced hitches like the MT, VT, french prussik, distal, etc., put the pulley right next to (in the middle of?) the hitch, allowing smoother and more complete slack tending (sometimes the weight of the rope will pull out slack for you). They are also faster and, in some cases, need closer monitoring. If you are still pretty new to climbing (just guessing since you aren't familiar with slack tenders) you might want to get a few more hours in your harness before stepping up to one of these methods.
k
