From The Document Foundation Wiki
- Click ( ▸ ) from the main menu.
- Chose which bar do you want to add a shortcut of the Additions Dialog and add it.
- Click shortcut where you add.
- Wait for the loading process (You will see the progress from the label at the bottom of the dialog.)
- Write the name of the extension which you want to install
- Click the Install button and wait for the installing.
- Restart the LibreOffice and the extension is ready to use.