The download button for each level is on the lower right of its listing:

You're going to copy the map file you just downloaded from the directory where Hidden Through Time saved it, to the directory where the game saves the editor maps you create yourself.

Download directory (Win10 default):
Editor directory (Win10 default):
Open the download directory and copy the map file from there to the editor directory. You can do this while the game is running, although you probably don't want to be in the editor menus when you do it (because they may not automatically refresh).

Pro tip: If you have a lot of files in your download directory, you can sort by modified date to find the most recent one easily.

From the game's main menu, choose "Map editor > Load map" and open the downloaded map.

When you click each objective in the objectives window, the editor area will pan/zoom to the object and show an animation to indicate the precise location.