You might try this reference

Don't do it you'll burn your house down and your insurance won't cover it!!! Stick to plumbing at least you won't take the chance of killing yourself or someone else. Don't play with electricity...

OP, the box you are looking for is the one where the wires from the breaker box goes to. The 'best way' IMHO to determine this will be to turn OFF the breaker. Go to one of the switch boxes, disconnect the all of BLACK wires and LABEL where they were installed. Make sure that and you are clear of the wires and turn on the breaker, then using a voltmeter determine which wire (there should only be ONE) is energized. If you get more than one black wire energized you are in the wrong box. Repeat above in the other box. Once you get to the correct box with ONE and only ONE energized black wire, your can go and install the new switch.
Now have stated the above, I strongly suggest you get at a minimum a handy man knowledgeable in this area or a licensed electrician to do your project.
Please be careful.

If the switch must be installed at the "load" end, VillageTinker's instructions tell you how to find the "line" (hot) wire, not the "load". "Load" means the light bulb. "Line" means the source of the power -- the panel. Whichever switch box did NOT have a hot wire connected to the original switch is the "load" side of the circuit, and that's apparently where your motion detector wants to be installed (although I can't think of a reason it would matter).

Here's a picture of a 3-way circuit, to help you understand how it works. See the two wires labeled "travelers"? The power travels from the panel to the lightbulb over one or the other depending on the position of the two switches. In the drawing, the light bulb is off. As you can see, if you flick either switch, the power finds a path over one of the two travelers to the light bulb.

But we haven't talked about "neutrals", which are the white wires. They are the return path from the light bulb to the panel (which I haven't shown in my drawing). Motion detectors normally need a neutral as well as a Line and Load because the gadget that detects the motion needs a return current path itself. If you mistakenly connect a hot wire (whether line or load) directly to a neutral (which would happen if you connected a white wire to the wrong lug on your motion detector) that's a "short circuit", which will trip a breaker and make sparks when you turn the circuit back on. If you accidentally did that when the power was already on (because you turned off the wrong breaker) , it would blow up in your face and the hot flying metal could ruin your eyes. Getting electrocuted is not the only way you can get hurt playing amateur electrician.

Like I said, I think you should get an electrician, but if you want to attach a picture of the instructions and the device, I'll see if I can decipher it for you.

Did the switch come with a wiring diagram that you could copy here or could you post the manufacturer and part number?

No white?