To make this work, your assistant needs to pull the cart toward you and your dog. If you are standing on the right side of your dog, then your assistant should pass to your right. This puts you between your dog and the cart which will make the entire ordeal as stress free as possible. After passing you, the assistant should turn away from you and return to pitcher's mound.

How did your dog do? Did he get up when the cart approached or show any sort of startle response? If your dog was very frightened of the cart you will want to spend some time allowing your dog to just sniff the cart and get better accustomed to it before proceeding. If there was a less dramatic response (he got up from sit but recovered quickly from the experience and showed curiousity instead of fear) then simply do a few more pass-by's in the same manner.

Once your dog shows no response to the cart passing by (i.e., he stays seated, but probably still watches it go by) then we can move onto the next step of this exercise. In this step, your assistant will pass-by you and your dog on your dog's side. This puts the cart much closer to your dog.

How did your dog do? If all is well, then let's proceed to the next step. If not, just take some more time to ensure your dog is comfortable with this experience (do not forget to reassure your dog also).