First check if you have configured JDK correctly:
- Go to File->Project Structure -> SDKs
- your JDK home path should be something like this:
/Library/Java/JavaVirtualMachine/jdk.1.7.0_79.jdk/Contents/Home
- Hit Apply and then OK
Secondly check if you have provided in path in Library's section
- Go to File->Project Structure -> Libraries
- Hit the + button
- Add the path to your src folder
- Hit Apply and then OK
This should fix the problem