At the beginning of each opponent's end step, if that player didn't cast a creature spell this turn, create a 2/2 green Lizard creature token.
5/1/2009 Predatory Advantage’s ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless the opponent whose turn it is didn’t cast a creature spell that turn, and (2) the ability will do nothing unless the opponent whose turn it is still hasn’t cast a creature spell by the time it resolves.
5/1/2009 Predatory Advantage checks whether a player cast a creature spell, not whether a creature entered the battlefield under that player’s control. If that player cast a creature spell but you countered it, for example, you won’t get a Lizard token that turn.
5/1/2009 In a Two-Headed Giant game, Predatory Advantage’s ability triggers twice at the end of the opposing team’s turn: once for each player. If neither of those players cast a creature spell that turn, you’ll get two Lizards. If one of those players cast a creature spell but the other didn’t, you’ll get one Lizard.