If you are getting "Install Desktop app", you didn't install the Desktop app correctly (permissions). Read 'Setup for Linux' on setup page (under activation code).

Download the Desktop app then open up a Terminal;

$ cd ~/Downloads
$ ls
expressvpn.whatever.deb <- copy filename
or
expressvpn.whatever.rpm <- copy filename

$ sudo dpkg -i expressvpn.whatever.deb
or
$ sudo yum install expressvpn.whatever.rpm

$ expressvpn activate

paste ur code

$ expressvpn connect

Now install browser addon.