Hi,
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.
