The Banco Nacion has two "tellers".... in the sense that you can usually exchange money before you exit the final customs secure area, when you exit the doors make a strong right... else, if you wander out of the second set of doors then make a hard right U turn and walk in the main entrance of the Banco Nacion branch.

I will repeat again.... do not use the crooks that you have in the baggage area... you can lose up to 15% of the value of your USDs / Euros when you change with these idiots.