Please first make sure that Mailbutler is installed.
1. Open Apple Mail.
2. Click Mail in the menu bar on the top left of your screen.
3. Go to Settings/Preferences > General.
4. Click Manage Plug-ins.
5. Activate Mailbutler.mailbundle by checking the checkbox.
6. Click Allow Access.
7. Click Apply and Restart Mail.
8. You will now see Mailbutler's features and the Mailbutler sidebar in Apple Mail.