I have a slightly different setup than you. I'm using the HA18 (3-way Switch) in a regular single pole installation. The diagrams from Intermatic show that it must be setup as receiving power from the breaker box and then sending it to your light. Well... my house wasn't wired that way. For me, the power goes to the light first and then to the switch. In order for the HA18 to have a proper neutral going back to the breaker box, I had to fish an extra white wire into the wall. It took a while, but at least it all works now. My other option would have been to connect the neutral to the ground but then that would have electrified the box and that just isn't smart.
So, who knows... you might have to fish an extra wire too.