In Sanctum, I need to collect 10 presents for the achievement "Festivus for the rest of us". I tried playing the first few waves of the free Christmas DLC, but I did not see anything like presents. So, where are these presents?
Look for normal sized Walker enemies wearing a Santa hat (I seemed to have one in each wave of normal Walkers), they will drop a single present when killed. Simply touch the present to collect it.
Additionally, after wave 4 (according to the wikia) on the Christmas DLC map, Santa will spawn and fly overhead. If you shoot him down you can collect those presents that he drops (I don't know how many).
I had to play the Christmas DLC level twice as a single player to collect 10 presents (I lost the first attempt on about wave 18, and only had to do the first few waves of the second attempt to get the last new I needed).