Posts from Might & Magic: Heroes VI – Buka: issues with the DLC. Auto-translated

Buka states that the issues with the codes may have arisen because "some users entered their DLC key as an activation key. Or, after the keys were canceled, they created a second account and tried to enter the key that they had previously activated in the first account." If you have a similar problem, you can notify Buka (you can find out how to do this here). At the same link, you can simply read the recommendations regarding actions with keys from Buka.