The minimum required version of Franz to run custom integrations is 4.0.0. You can download the latest version of Franz here.
To install the Trello plugin, download the latest release and extract it in the Franz plugins directory. You can access the plugins directory by going to the Franz settings page and finding the "Open the Franz plugin directory" option.
The direct paths to the plugin directory for the currently supported architectures are as follows:
Once installed, you will have to restart Franz in order for it to detect the new plugin.
This plugin is in no way endorsed by or affiliated with Trello (though Trello is awesome).