Starting with version 5.3 the Pro version comes in 2 flavours. The Admin version needs to be installed once on the main computer. On all other computers the non-Admin version must be used.
Click on the url for the server to download Valentina Server:
Double click on the dmg in the downloads folder:
And follow the instructions of the installer:
Now you only need to install the Admin version of Mail Archiver Pro. That will happen after you do a double-click on the installation app.