Setup

Google Play Music Desktop Player

For this skin to function properly, you must first install Google Play Music Desktop Player. After installation, you then need to navigate to the applications Desktop Settings, where you must then ensure that "Enable Playback API" and "Enable JSON API" are selected, as featured in the image below.

Desktop Settings

Troubleshooting

If you run into any issues throughout the setup proces, I would first suggest refreshing the skin and/or relaunching the GPMDP application. Though, if the problem persists, feel free to open an issue here.
Also, if you have an issue during the authentication process of the GPMDP plugin, I would suggest that you visit the Troubleshooting page of the GPMDP-Plugin wiki.