Motion paths in Particular can be tricky. Harry Frank brings you this overview of how to set them up, and a foolproof way to align the spline in 3D space with a simple expression.
Expressions used in this tutorial:
Particular Emitter Position XY:
pos = thisComp.layer("Motion Path 1").position.valueAtTime(0);
Particular Emitter Position Z:
thisComp.layer("Motion Path 1").position.valueAtTime(0)
You can also use this preset Harry created for AE CS4+, to use (with a try/catch thing so it doesn't error):