I do not know why I can not configure the Airnotifier access key in a MoodleCloud instance. The steps I carried out are the same I did in other plataforms:
1. Registration of the platform
2. From Site Administration > Plugins > Message outputs > Mobile Notifications I requested an access key. It seems that everything it is OK because the key has been retrieved successfully but the field is empty and in Messaging I can not see the column for Mobile notifications.