First, you have to import the project. To do that you just have to click Import project(Eclipse ADT, Gradle, etc.) and search for the project. Then browse and navigate to the project directory and click OK. Finally, open your android SDK Manager (Android Studio > Preference > Android SDK) and make sure that you have Android SDK Build Tools, Android SDK Tools, Android SDK Platform-Tools, Android Support Repository, Android Support Library, Google Play services and Google Repository installed.
When you buy the source code you will receive a documentation with more details. This documentation will show how you can import the code to the android studio and how to reskin it.