In the Steam version, you can access the dlc by going to the game files where there will be a directory labeled 'image set'. The Itch.io version probably does something similar.
Viewing post in Help (cant find the art pack)
From the DLC's description:
This Digital Art Pack includes:
· 55+ CGs
· Character portraits (including initial concept design sketches)
· Some in-game items
That sounds like a match. There should be 59 CG's in the 'CG' folder, the 'Characters' folder should have various sprites, drafts, and bios of each character, and the 'Items' folder should have images of in-game items. If you are missing the folder contents you may have a bad download, try re-downloading the DLC.