These 3-way kits are just a bundle. You can use the Master Switch all by itself and toss the Add-On switch. The Add-On switch allows for a 3-way switch experience.
It doesn’t wire the same way as a real 3-way switch. You need to be certain that the Traveler contact never receives Line power.
The Traveler is the connection from the Master Switch to the Add-On switch this Add-On switch simply triggers the Master switch to turn On/Off.
The big problem people have with these is the Neutral Wire. Many Older homes simply do not have a Neutral Line. However if you have an Bonded Breaker Box then you can use the Ground as the Neutral on the Master Switch only.
(PLEASE NOTE: DO NOT use the Ground as the Neutral for the light itself. This would be a fire hazard. In fact if you can avoid using the Ground as the Neutral at all it would be best). Also you should do your best to use the correct color wire in case someone in the future needs to open this up again. For instance if you use the White Wire as the Load Wire feeding the Light then you should Mark this White Wire with Black Electrical Tape on both ends of the wire in each box. If you use a different wire for NEUTRAL then you should mark this wire with White Marker/Electrical Tape on both ends as well. Try to use the Red Wire only as the TRAVELER/TRIGGER wire.
Main Load does not pass through the Neutral on the Master Switch. The Neutral simply allows the Z-Wave Wireless and built in relay to work. Picture below shows what Bonding is. If you have your Main/Master Breaker at the meter instead of in the Main Panel then the Main Panel is treated as if it were a Sub-Panel. I haven’t seen many older homes with this configuration though.
Some other brands of Z-Wave switches don’t even require the Neutral Wire they perform that through the ground.
Just be careful you verify how your 3-way is currently wired especially in older homes. If the Line comes through the light then to the 3-way switches this can cause an issue with your Z-Wave smart switches (especially if you are using a CFL or LED Bulb).
You should have your Line voltage going into the GE Master Switch coming directly from the Breaker Box and not through the light. If it is wired this way you will need to correct it first.
It can become quite confusing if you don’t understand AC Wiring. If you really just don’t know then you should contact a licensed electrician and have them install it. In many cases you will simply have to run some wire.
@zwavey shared a simple wiring diagram showing how it should be wired here: http://i.imgur.com/xeezvGC.png (Black line coming from Panel is LINE 120V, Black line from switch to the light is LOAD, Gray is NEUTRAL and RED is the TRAVELER/TRIGGER, the little caps are just demonstrating Wire Nuts you may or may not use the Wire Nuts exactly this way)
Ignore the GE specific Model Numbers. All the GE Switches I have seen that require a neutral wire the same way.
EDIT : Here are some more helpful diagrams: