I was trying to add Napa app for developing apps for SharePoint 2013 online and got the below message.
To get it enabled we need to have a Developer Site.
So we need to create a new private site collection with Developer Site as template
Open the site collection then à Go to Site Contents àSelect Add an App
Select SharePoint Store
Select Napa app from there
Login with your Hotmail account and add the App to the site collection.
Hope it helps.