Premium upgrades purchased directly through the Google Play Store app do not require a serial number to be activated.
The app automatically recognizes the purchased upgrade and makes all new features available without you having to do anything. The serial number input function in the app is provided for a different activation procedure.
If the app is not automatically running in Premium mode after you upgrade, the cause is most likely technical in nature.
To fix this problem, clear the app data and cache for both the Movie Edit Touch and Google Play Store apps on your device.