Seriously, what the hell, this thread was made 6 months ago and still no solution?
I have the same exact problem, I have a US Xbox Live account, I live in the US, I downloaded the DLC in the US on my US account, it's not region locked it's Ubisoft's screw-up.
I love them but 6 months and this is still not fixed, 6 months. =/
That's pretty messed up.
Actually, no, this is robbery.
Does ANYONE know how I can play my DLCs for AC2?