How do I install a dimmer switch?
Typically A dimmer that can be used in a three way circuit will have a common (black) wire, and two (non-black, non-white, non-green) travelers.  Here is a wiring diagram from the installation instructions of a random dimmer.


It can also be installed as the second switch in the circuit.


As you can see, in this example the common wire is black, while the travelers are red. Without further detail about your current wiring, it's impossible to provide more assistance.