assuming you have the xbox 360 version? if so.... and i'd assume you are looking for the free dlc that comes with the game/disc about the (free) dlc of the spec ops (first mission) (that you had downloaded) if so, from the main menu, select campain, then go to free-play and from there, there should be a choice to select one of the two faction (groups) (those being the U.S.S. or the Spec Ops) (obviously in this case you'd choose the Spec Ops and from there you'd be able to play the free first mission of the Spec Ops). if this is not the dlc you are talking about, then i've got nothing to contribute.
EDIT: i just had read in this thread of yours that you have the xbox 360 version, so...just read what i had posted above