New dimmer switch will not dim light - I am trying to hook up a dimmer switch in a electrical box that already contains a three way switch. I am trying to take power from the three way switch to power the new dimmer and light. The way it is hooked up now the new light works but will not dim with the dimmer switch.

First I'll ask you do you have a tester ? The first thing you must do is to determine the hot and neutral then it should be no problem from the hot you will connect one wire to the 3 way hot and one to the dimmer hot . Second connect the neutrals into the circuit. Right now it sounds as if the three way is controlling the dimmer

Was there a white neutral in that box too and how are you tapping in the neutral to the dimmable light?