Note: This information is for the live action movie, not the animated disc.
Start the "Special Features" side of the disc and select "Span Soundtrack". Press Right when "Trip Like I Do" is highlighted. Press Enter after the picture of Marilyn Manson is highlighted. Press Right on the next screen. Press Enter when the "Parental Advisory" graphic is highlighted to view a hidden Marilyn Manson music video.
Highlight the snake symbol on the "Parental Advisory" screen and press Enter. Repeat this a total of four times to summon a backwards speaking clown.