The disk image Drobo-Dashboard-3.4.2.dmg contains an Install.app that was apparently created with an unregistered demo of FileStorm and this demo has expired so I cannot install Dashboard. Please put a non-demo, licensed installer and repost.
Drobo Dashboard 3.4.2 for Mac DMG has an expired FileStorm installer
Turns out this is because I had an old copy of FileStorm on my Mac and macOS was trying to use this as the app to run the installer on the DMG. I removed the old FileStorm (rather than try to change default handler) and this fixed the problem.