The Panda Plateau Secret Level can be unlocked in Dingy Jungle. This is the first area of the DLC — and you may have to replay the level multiple times before the secret level puzzle appears.
To unlock, travel to the ‘Cross the Canyon‘ objective in Dingy Jungle and look for an optional, alternative path off the main road. If there isn’t a path here, or the path doesn’t lead to an entrance, you’ll need to exit and restart to regenerate the seed. If you’re lucky, a door will appear at the end of the path.
Inside, flip three switches to lower the barrier leading to the Panda Plateau scroll. Collect the scroll and you’ll be able to access the secret level from the zone select menu.
Panda Plateau is an especially good place to unlock for one reasons — it’s quick, and there are plenty of chests to grab. That includes valuable obsidian chests and 5+ gold chests. It’s easily the best place to farm for pure loot.