There is no polarity w/ resistors.

You'll always want to use the middle lead b/c that is the wiper, and gives you the variance. If you just use the outside 2 leads you'll have a fixed value resistor.

Which of the 2 outside leads you choose will depend on which way you want to turn the pot to decrease resistance/increase voltage. I set mine up so that a clockwise motion results in reduced resistance/higher voltage. Use your DMM. Always set the VR to max resistance before installing it, and it's a good idea to cut the unused lead off, so you don't get confused.