The MainWP Wordfence Extension requires the Wordfence plugin installed on your Child sites. To install the plugin, search for “Wordfence” in the plugin WordPress repository and install/activate it.
In your MainWP Dashboard > Extensions > MainWP Wordfence Extension page, you can find the list of all your child sites with the plugin installed. If you want to install the plugin to additional sites, the quickest way to do this is to:
Go to your MainWP Dashboard > Plugins > Install > Search page
Enter “Wordfence” in the search field
Click the Search button
Select all child sites where you wish to install the plugin,
Click the Install link.