The note refers to the note that Paul says he found with the game in Petscop 1.
The note is transcribed in the description of Petscop 1 (previously an annotation in the video, before annotations were removed), and reads as follows:
I WALKED DOWNSTAIRS AND WHEN I GOT TO THE BOTTOM, INSTEAD OF PROCEEDING, I TURNED THE RIGHT AND BECAME A SHADOW MONSTER MAN
6/13/97
For you:
Please go to my website on the sticker and also go to roneth's room and press start and press down down down down down right start
Paul enters the code in Roneth's room to access the Newmaker Plane.
In Petscop 20, Marvin is shown entering the code.
Theories[]
The first part of the note may have been written by Marvin. Marvin appears to the player as a shadow monster man in Petscop 6, later replicated by Paul in Petscop 9. The "glitch" itself is performed by walking down the stairs of the Flower Shack's basement side, and walking right at the very bottom before hitting the loading trigger.
The recordings shown in Petscop 20 suggest that the second part of the note was written by Rainer, for Marvin to perform when he was given the game.