Under the rear of the rubber. If you do a search - there are other threads that show how to figure out how much shim to use. Basically, you want the front of the carbide to be slightly off the ground. Look at your carbides - if they are worn in the front, you need to shim.
I run Curve skis with 10" carbides and needed to shim them 1/4" using a plastic shim and screwed it to the ski with stainless screws.