To unlock that scene you must have met the Rinkos, but NOT turned on Luna's machine (it becomes unavailable after that, although a future update will bring the scene back in at a certain point in the story). If both of those requirements are met, then you have to visit Luna's demesne at night, select "Look for Luna" and then "Check it out".
If you've already turned on Luna's machine, and don't feel like waiting for the update that gives you another chance at the scene, you can also use the "Gallery Cheat" on the laptop in your bedroom in the Manor to unlock all the gallery scenes :)