Another ci-en post, another part of the animation shown. The paywalled part shows a bit more again, with the bats changing Nako into a bikini that looks like the one succubat's wearing.
The entrance to the CG room is on the same screen as the starting house's save point. If it's not there it means you either don't have the right version, or you haven't beaten the ghostly boss yet, since I'm assuming she's a requirement (she's in the room).
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Not sure where you've gotten to, but if you've beaten succubat (5th warp point of area 4), take the only available route after her fight to the right. Keep going until you travel across a long bridge and keep taking the only open route until you end up in an area with mechanical platforms. From the next screen onwards everything'll be dark around you. Explore this area, pressing switches to open blocked paths, until you meet Siamia again and then beat the mech boss (it's not hard to find). Then follow the instructions in that post.
That's as much spoon-feeding as you're getting. If you haven't beaten succubat yet then go do that.