Some Mac users are reporting launch issues since the Hippo-Party-Plus Update. 

If you're experiencing similar launch issues, this workaround has helped other players:

1. Click on "Finder"
2. Click "Go" on the top of your screen
3. Click "Go to folder"
4. Type "~/Library/Application Support" in the popup
5. Scroll until you find the "Steam" folder, then click on that
6. Scroll until you find the "steamapps" folder, then click on that
7. Scroll until you find the "common" folder, then click on that
8. Find the Ultimate Chicken Horse folder
9. If you have it installed, delete it from your computer and then uninstall from Steam
10. Redownload from Steam

If these steps do not work for you, please send us a support ticket that details the error and let us know some of your Mac details.