Hey there! Since I'm not seeing any off these add-ons on the Xbox.com Marketplace either, my assumption is that these items are simply not available in your region. I'm sorry for the confusion here.
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These Mass Effect 2 add-ons not available in Xbox Live service,
only available in Cerberus Network
(which can be found in Mass Effect 2 main menu / Extras submenu, and requires Mass Effect 2 activation code).