The first Sliding Block Puzzle unlocks after beating main level #2 "Golem Getaway".
It will be a foxhole to burrow into with
in the center of the map, near the campfire.
(the bottom left foxhole)
To complete the puzzle, from the start:
Hit the right fox down, left, down, and right.
Hit the left fox down, right, and down.
Hit the right fox up, right.