Have you identified your wires? If not, I recommend doing so before attempting to install the dimmer. The red and brown leads coming off the dimmer are for your traveler wires coming off your 3-way switch. The black lead should feed directly to your load wire, coming down from your fixture. Be sure to follow the wiring diagram below. If you are unaware of which wire is which, I recommend using a voltmeter or similar device. Otherwise, I woud suggest hiring an electrician.