Follow these simple steps to start using Flatpak
Flatpak support is built in from MX 18 and later. It is only required to activate the Flathub repository following these instructions:
Open MX Package Installer (open the menu and look in MX Tools), select the "Flatpaks" tab, to activate the repository you will need to enter the root password.
To complete setup, restart your system. Now all you have to do is install some apps!