The maps should be available on the Mercenaries mode; some people are recommending rank B and over on the first three maps, others and A. I can barely scape a C in Resident Evil 6's Mercenaries, myself so I can't answer with confidence. Someone who has beaten the game might be around soon, though.
All maps (the regular 3 and the other I bought) are visible in the merecenaries mode, apart from the 2 from map pack A (Requim of war and Liquid fire). They are "?" It's frustrating. You pay and receive a "?" instead of two stages...
Do you have a friend with those maps? If so, get them to invite you to a game on one of the maps you can't access. If it says "not all of the players have the maps downloaded for access", then you know it's a download issue and not an "I haven't unlocked the map yet" issue.
lets assume its an installation issue. is it possible to reinstall the maps? that was my first purchase in the store, so I have no experience with reinstallations. are the maps now in my account for ever or how is it working?
DLC isn't exactly part of the game, but an extra program that is compatible with it. You can delete your DLC and re-install it without having to uninstall your game as well. And, if you make the purchase, your account will remember it on all options.