MODX comes with its own web-based package management. If you want to install supported addons and plugins, you can do it with the MODX Dashboard.
Navigate to “Extras -> Installer”
Click “Download Extras”.
Select your extra and click “Download”.
Then, click “Back to Package Management”.
Click “Install” and then “Continue”.
If the installation is OK, you will get a screen that says “Successfully installed package