Rename one of your park guests (or more) the following codes to get the desired effect:
While playing, click on a peep in your park.
Click the General Information button (has an "i" on it).
Click the peep's name, then delete or backspace over it.
Enter the cheat code as the name, then press Enter.
The cheat will take effect.
Alistair Lindsay - SoundWarp
David Walsh - Unknown Effect
M Brookes - Underground SI
Mornington Crescent - Allow Path Walls
Rick Griffiths - Tunnel Shark
The original Rollercoaster Tycoon 3 cheats also work with the expansion:
John D Rockefeller - Add $10,000
Mouse - Guests Stare at the Ground
Atari - All Guests Laugh and Cheer
Frontier - Rides/Coasters Never Break Down
Guido Fawkes - Advanced Fireworks Editor
Chris Sawyer - Guests Jump for Joy
Sam Denney - Peeps Ride All Coasters
Jon Roach - Peeps Ride All Non-Coaster Rides
John Wardley - Removes Height Restrictions on Coasters
James Hunt - Buggy Rides Through the Park
Shifty - Guests Dance
D Lean - Flying Camera Route Editor
ATITech - Speed Up
Jonny Watts - PeepCam
Atomic - Big Explosion
A Hitchcock - Unknown Effect
PhotoStory - Unknown Effect