| You can't use that kind of dimmer in this situation (at least not practically). You need to find a 3-way compatible dimmer. You should be able to find these at the home center. There are essentially two kinds. One kind, just replaces one of the switches. The other switch just turns the light between off and where the dimmer is currently set. The fancier kind are sold in sets and allows dimming from either end (one will typically be a master and the other a remote). | 
