How do I wire a 3-way motion sensor?
Depending on the device you're installing, the installation can be a bit different.  Consult the installation guide that came with the device, to determine the proper wiring.
I've randomly selected installation guides from a couple available devices, and I'll show the differences.  Keep in mind that your device may vary, and you should consult the documentation with your device.
Lutron
The Lutron installation guide, shows this wiring diagram.

Which seems very straight forward.

Connect the Black (common) wire to the Black (common) wire from the device.
Connect the Red (traveler) wire to the Blue (traveler) wire from the device.
Connect the White (traveler) wire to the Black (traveler) wire from the device.

Leviton
The Leviton Installation Guide, shows a bit more complicated diagram.

They even provide a troubleshooting section, to help you get the thing working correctly.


NOTE:
This is just a small sample of the available devices, consult the documentation that came with the specific device you are trying to install for proper installation instructions.