Alternatively, you can go to System Preferences - General.
From there, unlock the lock at the bottom of the window and "Allow apps downloaded from: Anywhere." This second method is the preferred method, as you will no longer be bothered for future XBMC updates.
This of course disables scrobbling and any synchronisation.
The problem is apparently due to the API changes, their Twitter account confirms that the API has changed...Before getting started, you’ll need to jailbreak the Apple TV2 with Seas0n Pass, Seas0n Pass is the easiest jailbreak utility around, but you could use Pwnage Tool or Redsn0w if that floats your boat.After you’ve done this, here’s a rundown of what you’ll need to do: Your Apple TV2 web server should now be up and running, this can be verified by pulling up the ATV’s IP address in a web browser.If you want the webserver to automatically start when the Apple TV boots or reboots, you’ll need to add a launch agent plist to /Library/Launch Daemons/, plist files are different than bash scripts in that you have to spell out each individual flag and argument as a string.This page contains most of the questions that I get including the correct answers. But it changed to a Video Add-on only in version 3.5.0 (See this post).