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You have to dig through the new add-ons section to find it. You can not search for it, nor can you just go to Isaac's game page and find it through there. You'll have to go to the PSN store, select add-ons, then "new", and scroll though until you find it. It's buried under a mountain of digital currencies and costumes for games that exist only to sell you costumes, but it's there.
There's now a board for discussing all things YouTube. http://www.gamefaqs.com/boards/1323-youtube-general