Get on Xbox Live and get the program called Xbox Live Labs.
To get you free avatar DLC just simple let it run for set amount of time that is all you have to do.
Here the running time(s):
That Is It. Do That And You Will Get All Three Pieces Of Avatar DLC.
Just Thought I'd Spread The News. So Enjoy, And It Only Last For A Limited Time(Think March 27(ish).