Puzzle Maker comes with Puzzling Places and is accessible to anyone who owns the game through the Steam client. To download Puzzle Maker, open your Steam Library:

If you don’t see Puzzle Maker listed there, it’s because Puzzle Maker is a tool, and tools are hidden by default. Click the Games and Software drop-down:

And enable Tools in the drop-down:

Puzzle Maker will now appear in the list. If you have a lot of items, you can also search for Puzzle Maker.