you can only have one dimmer, the other switch needs to be not a dimmer. the green wire is ground, it connects to a bare copper wire found in the box, or to the box itself, if the box is metal. the black wire is"common" it either connects to the wire found in the box that has power in it or to the black wire that goes to the light. the 2 red wires are"travelers" they connect to the wires found in the box that go from switch to switch. if you don't know how to check which wire does what( continuity), it is possible to keep changinging the connections until the switch works. you will want to do this with a conventional switch, and install the dimmer afer you identify what wires are travelers and wich are common. you will destroy a dimmer switch by searching for the right connection. once you have identified the common wires, it will be on the black screw of a 3way switch; you can put that wire on the black wire of the dimmer. one red goes on one traveler, the other on another traveler. there might be a white wire in the box, if it is connected to other white wires and not to the switch, leave it alone. if a single white wire is connected to old switch, it may be a traveler.you will connect a red dimmer wire to this. opening the other switch location will show you if the white wire is used as a traveler.the black wire has to be either power coming in, or the wire going to the light. the reds are just connecting the 2 switches.
If you have one wire into your switch box for your light. this is called a switch leg, the dimmer should be wired across the black and white wire wires.
Sounds like you are are installing a single pole switch (your load is controlled from one location only). If your existing switch is hooked up to a black and a white wire you are dealing with a switch loop and that white wire is being used as a hot wire. Make sure the circuit breaker is off of course, and hook up the black wires on your dimmer to the existing black and white wires on the original switch (polarity doesnt matter here) the green wire on the dimmer is a ground and should be hooked to a bare copper wire or green wire in the box if it is there.
The black and white (neutral) wires connect to an unswitched voltage supply. The red wire and the same white (neutral) wires connect to the load.
The two black wires go to the other 3-way switch. The red wire goes either to the load or to the power, and the red from the other 3-way switch goes to whatever's left.
If the dimmere only has 2 black wires and one green, it's not a three way.
Not enough information, but generally: you can't.The existing switch location will require another conductor, at the very least.In other words, the existing switch will need a different cable and the wires will need to be connected properly to the new 3-way switch.
It sounds like your switch box is wired for a four way switch. Not a three way. Can't be positive without looking at it. Are there two other switches that control the same light? If so the dimmer would have to go in one of those boxes.
If the switch has a black screw this is a 3way switch. On a 3way circuit on one switch you connect the black power wire to the black screw, and on the other end of the 3way circuit on the other switch you connect the black wire coming from the light to the black screw.
This is a relatively simple wiring job. Remember that all a switch is, is a means of interrupting the power coming from source (electrical panel) and travelling to load (light fixture). Without the switch, the light would stay on constantly. The black wire is a power wire and so is the red. The white wire is a neutral wire, and the green wire is the ground. You may have to do some trial and error to get it right as I can't see the actual wiring diagram you have. After shutting power off to that circuit, disconnect the dimmer switch and remove. Connect the two white wires together with a wire connector. Connect the two green wires. The red wire was probably part of the dimmer switch and should now be gone, but if it isn't then just cap it off. Now connect the black wire from the fixture to one of the terminals on the switch, and connect the black wire from the electrical panel to the remaining terminal on the switch. That's it.
with a standard 1 pole light switch (one switch operating the light) it is black wire to black wire and white to white (non grounded)