This is a Joomla plugin, install it on the page Extensions => Manage => Install
Go to the Upload Package File tab, and upload the downloaded package
Once installed, publish the plugin in Extensions => Plugins
When accessing the plugin's configuration page from Extensions => Plugins, you will see the following screen:
Group: The Joomla icon system groups the quick icons, by default the AcyMailing icon will be placed in the "Extensions" group but you can choose any of them.
Icon legend: This is the text displayed next to the icon. If you don't specify anything, the text "Newsletters" will be used
Here is what you should see on a Joomla 3 website once installing and publishing this plugin: