In the boardwalk level where the sniper and shotgun respawn, where the top of the main stair area is, there is a rock on a 45 degree (it is advised that you have sprint on).

Run up it and jump onto the grass area, to the left of you, you will see a tree with a Y shape in it.

Jump into it (this might take a couple of tries) and then jump up to the top of the tree and then jump straight ahead and you're on the platform. i

If you're playing infection the zombies can jump up others ways so watch out.