I believe the free-to-play thingy is at the bottom of the Uncharted 3 page (on the PS3 Playstation Store). I think it is under some Other or Misc. section. Anyway, scroll to the bottom, it's around there somewhere, right next to the soundtrack.
i have the free to play version of the game but my brother has the cd and my other bro has the free to play, seems like i can only join those who have the free to play but one time i joined my bro one time an he had the cd, 9/10 its not the internet because the same thing happen to me with injustice i think its super lame i cant join my brother in a match because we have different versions of the same game -_- hope naughty dog comes out with an update