The scroll is in the far room of Henrik's apartment (3d floor, on the right), next to the CD collection, accessible after you find all CDs.

 
You need to find all cats on the poster.
The problem is that sometimes it's quite difficult to spot black cat on a black tree, especially if cat is not there.


 
The tree is likely a reference to cover art of "Thulcandra" demo by Norwegian black metal band Darkthrone. You'll receive Thulcandra achievement upon completion.