App will not run on site page

I've created a simple App using Napa (SharePoint online trial). When running it it displays the text as expected. Then I've published it. In my Team site I add a page in Site Pages library and add the App Part. An App Part appears with a blue background with the text 'Your conent goes here ...' . It should say Hello World. In Napa I've set permissions but seems to make no difference.

Any Ideas?