Well first of all, the documentation on line for Office 365 is wrong, the Office 365 is not in Visual Studio 12 RC natively. To be expected for a beta version.
To start building these apps you will need to sign up at NAPA and then follow the basic app to make sure everything is working.
Sign up for an Office 365 Developer Site
How to: Set up an environment for developing apps for SharePoint on Office 365
Getting started now will put you into a great position to sell and make money off of developing apps for Office 2013, Visio and other products.