The community is also a great place to check out. Try this link for help from other users:
Please review the Third-Party Game Support topic to determine whether your question should be directed to the game developer or to Steam Support. Sending a support request to the wrong party may result in a delay in processing your request.
Retail CD-Keys for this game cannot be registered through Steam. If you wish to create a shortcut to this game under the Library tab, instructions can be found here:
If you're encountering issues when launching or playing RollerCoaster Tycoon 3: Platinum, make sure everything installed correctly.
If you are receiving the error "Failed to start game (app already running)," your game has silently crashed or failed to launch. Many potential solutions can be found in our article regarding this specific error:
If you continue to encounter this error, troubleshoot the crash/failure to launch with RollerCoaster Tycoon 3 support.
If the above does not resolve the issue and you are running Windows 7 or Windows Vista, please complete the instructions in our Windows 7 and Vista Troubleshooting FAQ.
If you are running in Windows XP, take the following steps: