To install Posterno via FTP - please download it from here first: https://posterno.com/download. This will download a zipped package to your computer.
Once you have the zip file, open it (typically by double clicking it) and inside you'll file a folder called posterno.
Use your favorite file transfer tool (e.g Filezilla) to copy that FOLDER (not just its contents) to your server, and place it in folder
Then go to the plugins page in your WordPress site and you should see Posterno in the plugin list. Click the Activate link and you'll be ready to use the plugin.