I was wondering what Justin meant by "Major chord" as well. I've tried it as a power chord with a 5th string root, but also as a A shaped Major barre chord too. They both sound pretty good to me. In the transcribing section of the website, it says power chords, here's the address:
http://justinguitar.com/en/TR-020-SongTips1.php But like I said, both sound good, I guess it's just whichever your ear determines you like.