Brian, open the hypershade and click on the material node with the displacement map on it. Then, click on the SHOW INPUT / OUTPUT CONNECTIONS, this will map out the connections in the lower window. It will show you the displacement node. Click on it and you should be able to change the values.

What you have is looking good. But edit it so you have varying levels of displacement. Also, watch the 'mountains'. If you notice, the peaks look good, but the sides are all smooth. Actually, by adding a bump map to the displacement shader, you might be able to pull off bumpy sides.

Oh, and yes... the displacement shader takes a while to render. You might want to render it seperately from the truck and composite.