Game modifications or custom content are not supported by Steam Support. Steam Support will not be able to assist you with any issues that you encounter when installing or playing this modification. Any questions that you have should be referred to the Technical Support portion of the forum that is linked above.
The community is also a great place to check out. Try these places for help from other users:
If you're encountering issues when launching or playing Zombie Panic: Source, make sure everything installed correctly.