Installing Anki add-ons is not too bad, but it's slightly different between versions 2.0.x and 2.1.x.

For background, I started here with this Low Key Anki guide and wanted to install one of the provided plugins (ResetEZ.py) but it didn't work out of the box.

Add-ons, in OS X, are found in $HOME/Library/Application Support/Anki2/addons21. Make a directory, I named mine resetez. Inside, add a file named __init__.py. Take the contents of ResetEZ.py found on the link above (maybe a page or two ahead) and put it as the contents of __init__.py. Read the instructions and configure. Restart Anki and you should see a new menu item under Tools!