This page of our guide to Psychonauts 2 lists the Half-A-Minds available in the PSI King's Sensorium.
You can find 4 Half-A-Minds in this level.
You'll find the secret under the wooden path. Just run under it, then burn the poster that blocks the way further. The Half-A-Mind is located at the very end of this area.
Half-A-Mind No. 2
Location The Eye Shrine.
This Half-A-Mind is hidden on a wooden platform, right under the windmill. You need to slow down the windmill before you can jump to the collectible.
Half-A-Mind No. 3
This Half-A-Mind is at the end of the rainbow path, near the end of the Concessions stage.
Half-A-Mind No. 4
This secret is located on a small rock platform, right next to the waterfall.