You install Joomla administrator modules and plugins just as you install regular Joomla modules and plugins.
Installation
Navigate to Extensions > Install/Uninstall
3 Methods of Installation
1. Upload Package File (most common method)
Browse for the zip or tar file of the extension.
Click Upload File & Install
2. Install from Directory
If the file exceeds your php file upload limit on your server (usually 2mb on shared hosting), you may need to use this method.
Unzip or untar the file locally (or upload the zip or tar via a Joomla file browser like NinjaXplorer and unpack) and upload to your /tmp directory in a new folder (ex: /tmp/myextension)
Enter the path to this file. Joomla will autofill the path to your /tmp directory for you, so you just add the new directory name.
Click Install
3. Install from URL
If the file is available online (usually free files) and you don't want to save it locally, use this method.
Copy the URL of the file location.
Paste in Install URL.
Click Install
