What we see in that scene is basically a bit of a funny setup for a possible sequel that doesn't really have much bearing on the actual plot and only tries to end the movie with a notion of "On to new adventures!" and wasn't part of the theatrical release (as pointed out by Oliver_C in his comment and as Louis Leterrier confirmed in an interview).
To recap it a bit again, it puts the Horsemen into a new location where they find crates containing what is supposedly their gear for the next job. When posed with the question how to open those crates,
Henley: Guys, the cards.
Merrit: Wha...What card?
Daniel: Sorry, what do you mean, "what card"?
Merrit: (fumbling in his pockets)...Okay.
So the point here is that Henley seems to be referring to the tarot cards each of the Horsemen got as their invitation at the beginning of the movie, and which were later used in the tree at the end when they met Dylan Rhodes (or rather Lyonel Shrike's son). The joke here is, that Merrit actually forgot to bring his card. There is not really anything more to it. Those cards served as their access into The Eye throughout the entire movie and are likely used in a similar way in further tasks and interactions with The Eye, like accessing the crates with the gear for their next show/adventure. You could say they're somewhat like the Horsemen's ID cards inside the organization of The Eye.