Return the thing to the retailer and tell them your ARR copy is of a different region and thus, won't accept the expansion. It was a simple mistake and you had no idea your copy was regionally different.

And no, if it is a region locking issue, the agent you contacted wouldn't be able to help you, as, unsurprisingly, they can't give away free copies of the game if you, the player, bought the wrong version.

It's no different than complaining your PS3 copy won't work on PC.