Airmail allows you to install and use plugins developed by third-party developers. Currently, such plugins must be manually added to the appropriate folder as explained below. (NOTE: if you have not installed any plugins, this window will be empty.)
To install a new plugin, click on the ‘Plugin Folder’ button shown in the screenshot below:
This will launch Finder and if you do not already have plugins installed, the Finder window will be empty.
After this finder window opened, copy the plugin that you downloaded in this 'Plugin Folder' and now, go to 'Plugins' and reload the plugins and then enabled them.
Following is how the Plugin Window appears in Airmail. Go to Menu > Window > Plugins.