Moving Over Terrain with iTween

Hi, I'm trying to move an animated creature over a terrain. I've been using iTween to do the waypoint patrolling, but I'm having trouble making the creature track the Y height of the terrain. Ideally I'd like it to follow the orientation of the terrain mesh, but at this point I'll settle for just tracking the Y.

What I've got is this:

function CheckGround(){
var hit : RaycastHit = new RaycastHit();
if (Physics.Raycast (transform.position,Vector3.down,hit, 2)) {
    myRat.position.y=hit.point.y + 5;
}       

}

which I'm calling from Update().

function tween(){
iTween.MoveTo(gameObject,{"path":path,"time":mySpeed,"orienttopath":true,"looktime":.6,"easetype":"easeInOutSine","oncomplete":"complete"});    

}

which is called from Start();

path is an array of Transforms

So, my understanding of how this should work is that iTween fires up when the game starts and moves the creature through the waypoints in the path. Every frame of that, the creature raycasts downwards and, if it hits an object or the terrain, it will adjust its Y access to be the same as the point where the ray hit plus 5.

The creature is on the Ignore Raycast layer. The waypoint part is fine, but the Raycasting doesn't do anything, in fact the creature sinks slightly in the first few seconds. The size of the terrain is 500x500.