Google had unveiled the Material Design at the San Francisco event of the Google IO last month and it will be implemented in the upcoming Android L operating system. Google has already implemented the same design for the Google Play App, which has started rolling out. So if you have your new Google Play installed (v4.9.13), then you should be able to observe the new interface.
The Google Play update app will pushed smartphones slowly and many smartphone devices will get the new update after a longer time. However, if you dont want to wait and want to get your hands on the new Google Play App, click on this link and download the APK file. Then simply install the app and enjoy the new eye-candy feature.