If you cut out the treat, then you're really diminishing the power of the click. The click is not the reward - it does not, in and of itself, reinforce the behavior. The click is the secondary reinforcer, and it marks the behavior - it says to the dog, "Yes! That's what I wanted!", but it doesn't do anything to make the dog more likely to repeat that behavior. You need to pair the click with a primary reinforcer (it doesn't have to be a treat, it could be praise or pets or a toy or a game or whatever the dog finds reinforcing - treat is just shorter and easier to type), so that the dog learns not only what you want, but what's in it for the dog to do what you want.
Much as we sometimes hate to admit it, dogs don't work because they love us; they work because it pays off for them. This may be in terms of food/treats/toys/games, or it may be in terms of affection, or it may be in terms of not getting yelled at - the point is, doing what we want must be more rewarding for the dog than not doing what we want. If you're trying to distract a dog from chasing a squirrel, for example, chances are pretty good that giving him a pat on the head just isn't going to cut it.
Here's a good article on basic clicker training (there are a number at ClickerSolutions, as well):
http://www.pawsitivesolutions.net/behav ... ining.html
For the down, I wouldn't even use the word yet. Work on teaching the behavior first - words will only confuse things at this point. Down is a hard one for dogs, so start with baby steps; if you're teaching down from sit, first click/treat for any downward movement of the front half of the body; then for a bend to the elbows; then for more movement toward the floor; then for elbows on the floor; then finally for chest on the floor. You don't want to increase criteria too quickly - generally you want at least 80% reliability before you move up. Once you have the full behavior, at an even higher reliability, you add the cue.
The technique above would be called shaping the down. You can lure the down - I've never really been able to do this correctly, but other people can!

You hold the treat in front of the dog's nose, bring it slowly down to the floor, then slowly drag it forward until the chest hits the floor. Or, you can capture it - keep a clicker and treats handy, and whenever the dog lays down, click/treat. Soon he'll start offering the behavior in hopes of a c/t.
