1-You need to have the Thrillseeker Edition
2-Go to your Steam folder in your main directory , then : Steamapps/Common/Planetcoaster/.......and you 'll find "the Digitalsketchbook " and the "Soundtrack"
Have fun!
As gecaum said if you have Thrillseeker Edition but you can also find the files as follows
1 - Open Steam App on Windows.
2 - Right Click on Planet Coaster and Click on Properties.
3 - Select Local Files Tab and Then Browse Local Files.
4 - You Will Then See The Folders and Files For The Extra Content.