There are several ways to install a Drobo App onto the Drobo 5N.
From your host connect to the DroboApps share using explorer for Windows, finder for Mac, and command-line if using Linux (e.g. afp://[DroboIP]/DroboApps.) If you get prompted to authenticate to this network share, you should provide the Drobo Dashboard admin username and password.
Once connected to this share, place the Plex.tgz file to be installed onto this share.
Notice that nothing has happened to this file, this is because the Drobo hasn’t been instructed to install this application. If you already have access to the Drobo via SSH, you could just execute the following command
/bin/sh /usr/bin/DroboApps.sh install
If SSH access is not yet enabled, a reboot of the Drobo should be sufficient. Upon boot, Drobo will look into the DroboApps share to see if there are any applications that need to be installed. Once the unsupported Drobo App is installed, it will show up in Drobo Dashboard.