I had ported a project from Eclipse to Android Studio and one of the things I needed was an assets folder. I keep encryption keys in it but there is no assets folder by default in Android Studio.
Solution: Right-click the module (e.g. app) and choose New > Folder > Assets Folder and it will put it into your project and into the correct place on the file system. If you want to find that place, right-click the folder after creation and choose "Show in Explorer" or whatever it says on other platforms.