The Museum is unlocked after completing the campaign on any difficulty. The Museum does not have a difficulty selection; however, it will load in on whatever difficulty you last used in the campaign. Load a mission from mission select on Recruit difficulty and then head back to the main menu and select the Museum. There are two rooms filled with enemies who are inanimate until you push the red buttons found on the reception desk of each room. I recommend heading to the second room and grabbing two LMGs then activating the button in the second room. Funnel the enemies out of the room and through the hallway as needed. Resupply your weapons and then head back to the first room and repeat.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!https://www.youtube.com/playlist?list=PL-HeW6XkKdUK97oYVSMU7...