http://blog.lekman.com/2012/11/sharepoint-2013-sideloading-of-apps-is.html
Solution: You need to enable the Sideloading feature on the site.
1. Download and install the SharePoint Online Management Shell for PowerShell
2. Download the script Sideload.ps1 and execute it within the SharePoint Online Management Shell.
Note: This is not recommended for a production site. Deploy applications using the App Catalog on a production site.