Memoria Misera EX is a new enough end-game content that it still appears in the trial tab of the Raid Finder, not the trial tab of the Duty Finder. Click on the "High-end Duty" tab at the top of the duty finder to get there. Alternately, you can get there directly through the Main Menu --> Duty --> Raid Finder. I have that hotkeyed as Ctrl-U but I'm 90% sure that's not the default hotkey.