Is this the Cinnamon applet store?

The web version, yes. They can be installed from System Settings > Applets. You can also get there by searching the main menu for Applets.

Where are the applets stored? I extracted the .zip, didn’t put it in the right place.

You don’t need to download it from the Cinnamon Spices website, they can be downloaded from the Applets in System Settings like I mentioned. See my screenshot above. :backhand_index_pointing_up:

Files are stored in (3) places:

~/.cache/cinnamon/spices/applet
~/.config/cinnamon/spices
~/.local/share/cinnamon/applets

As @Yochanan already posted, it’s much easier to use the Applet.