Napa was created with the objective to facilitate a 'lightweight, in-browser experience' to build SharePoint or Office web app in the same browser where they would run. Ergo, it was designed to kind of complement the inherent features of the browser.
For advanced tooling, one would have to turn to Visual Studio 2012.
Unfortunately, at now, the 'Napa' is only for development and deployment packages, no resource debug.
I hope you understand. Maybe who knows what the future may arise this
You can download Project Napa(the format is filename.exe):
Execute file downloaded:
Wait for some seconds, will open vs 2012:
with VS 2012 opened, set connection to online.
Usually they come pre-configured, after do made last step, break point in your code and start mode 'debug'. :)
sorry my bad english.
hope this help.