How to modify a tex2d offset in a cg shader?

I have a modifyed terrain shader, but I'd like to apply a normal on it. the problem is that I modify the texture tile x and y, but it gives me horrible results. So, How can I change offset in a cg shader ??

PS I can't access the material, but I can only change this part of my shader:

 norm =  UnpackNormal(tex2D(_BumpTerrain, IN.uv_Control));

come on, any hint?