The add-on needs to be opened from inside Google Docs,.
You can watch this video or keep reading:
The first step is to open a Google Doc.
- Click "Add-ons"
- Click "Kaizena"
- Click "Open Kaizena"
Here's what that looks like:
The first time you open Kaizena, you'll see a message saying "A script attached to this document needs your permission to run." - click "Continue."
You'll see a popup asking you to select your account: if you have multiple Google accounts, make sure to select the account you use for school. Kaizena will ask for permissions - click "Allow":