Once you're set up with an account, you also need use Construct 2's 'Export for AppMobi' option, and export over the existing AppMobi project (by default in Documents). Then, if the XDK is already open, click the 'reload project' button *twice* (a few seconds apart) to see the change. You have to reload twice because of Construct 2's offline caching: the first reload will see the update and download it, but keep using the old version, then the second reload starts using the update.