Hello! If you have pre-ordered the game, the exclusive content should have been sent to the email address bound to your Itchio account during release during the end of March / the beginning of April.
If you have not received it, please send us an email to customer.niftyvisuals(at)gmail.com either using the email that you purchased the game with / the one bound to your itchio account, or the receipt of purchase so we can locate you. Thank you so much!