Graphical way to install .flatpakref (Flatpak) files?
Ubuntu 14.04
Based on burhanuddin abbas comment there's a problem on Ubuntu 14.04.
Unable to locate package flatpak , because it is not supported for ubuntu 14.04 . Visit here for more details
Newer Ubuntu versions (tested on 16.04)
1. Installation of flatpak
sudo add-apt-repository ppa:alexlarsson/flatpak.sudo apt update.sudo apt install flatpak.
*1.1. Adding the flathub repository
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo.
2. Installation of MonoDevelop with flatpakref file
flatpak install monodevelop.flatpakref.
3. Running the application
flatpak run com.xamarin.MonoDevelop (probably, not tested, based on @purlantov answer).
Source: http://www.omgubuntu.co.uk/2017/07/7-flatpak-apps-can-install-right-now-flathub .