Hold your finger on the screen to open the Circle Menu.
Sweep your finger to PREFS
Tap "Where's my music?"
Tap + icon in upper right corner.
Select Google Music
If you have more than one Google account defined on your device a window will popup. Select an account that has access to Google Music.
Change the view to newly created account (if not changed automatically). Tap circular arrows that are left of + icon.
Notice the notification area! A small warning sign shows up! Open notification drawer and you will see xenoAmp is asking for "sj" service access. Tap the notification.
Allow xnoAmp to access sj service (sj means SkyJam - old name of Google Music)
Go back to "Where's my music, select Google Music account and tap circular arrows again. The synchronization should start in the background.