The Spikes aren't PWMs, they just use a PWM cable. You can figure it out looking at the program aliases. At any rate, on the relay's PWM cable, black is black/ground, red is reverse, white is forward. So you'd connect the red cable to the white pin of a second digital IO.
Here's IFI's info on it:
Spike FAQ
Spike Manual
EDIT:
As it says, you need to get 12V of power to the spike, so no using 7.2V batteries.