Just installed a 3 way dimmer for one of the two switches but it only works when the other switch is in the on position, did I wire wrong or does the other switch also have to have a dimmer installed on it???
More than likely you connected it wrong. Standard three way dimmers should only be dimmed at one loctaion. Look at the house wiring for the dimmer location. If the three wires for the dimmer originate from two seperate cables coming in to the box, the two wires coming from one cable are called the "travelers" and should be connected to the like colored wires or screw terminals on the dimmer. The single wire coming from the other cable is the "point" in the circuit and should be connected to the odd colored wire or screw terminal on the dimmer. If there is only three wires coming in to the box you will need a voltage tester. Remove the dimmer. Check each house wire for voltage to ground. One will be live. Flip the three way switch and recheck the wires for voltage. If the same wire is still live this is your "point" (common) wire. If a different wire is live now then this one and the other one that was live are the "travelers".
If you only removed the 3 wires on the dimmer then you connected them wrong to the new dimmer. Read the directions and determine which wire of the dimmer is the common. Then switch it with one of the other wires. If that doesn't work switch it with the other wire. If you removed more than the 2 wires on the dimmer tell me what is in the box and what they are connected to.