The plugin can be installed entirely from WP Admin or Manually, similar to any other plugin.
From WP Admin:
1. Go to WP Admin – Plugins – Add New
2. Search for “mplus intercom”
3. Install the one from 79mplus, then Activate it
1. Download the plugin zip file from here
2. Extract it
3. Upload the plugin directory on wp-content/plugins
4. Activate Mplus Intercom Subscription plugin from WP Admin – Plugins
Please note that you would also need to configure it in order for the plugin to work.