Because I searched for I don't even know how long and couldn't find an answer to this question...
The short version — do this:
The long version:
You're in the US, you buy some Call of Duty map pack expansions for XBox 360 from EB Games there, you get back home to Canada and try and redeem the codes, and what happens? "This item is not available from your region." FFFFFFFFFFFFFUUUUUUUUUUUUUUUUUUUUU-