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Found on xbox media center forum: [AppleTV] Launcher 3.2.2 and ATV XBMC nightlies released - XBMC Community Forum

SSH into ATV.
run command:

defaults write com.teamxbmc.xbmclauncher XBMCAdditionalDownloadPlistURLs -array-add http://mirrors.xbmc.org/nightlies/osx/update_urls.plist