While out on patrol with Dina, you will come across a moose that has been killed. You will head inside a supermarket to kill off any infected that have made their way into this area. The supermarket has quite a few infected to kill, but also some puzzles to get through.
After killing a few infected, you will find yourself in the loading bay out the back of the building. Dina will attempt to open up a metal gate that truck trailers load through but it is locked shut. From here, you need to try and find a way into the supermarket. Here is what you need to do.
Hold down the crouch button near the truck trailer to go prone. This will allow you to crawl underneath the trailer and access the area on the other side.
Once you are here, you will find some loot inside of the trailer, but find yourself stuck again. Even the hint this area gives you is a little confusing.
Climb up on top of the truck cab. This is the blue part of the truck that the driver sits inside. There is a gap between this and the trailer. If you sprint and jump, you will work up enough speed to be able to jump over to the other side. Once you are on top of the trailer roof, you will be able to run over and access the supermarket.