We've clarified some of the points in the article below. Hopefully this should answer any questions that you may have.
The 14-day Buy and Play program was a special offer where gamers who owned the Halo 4
Standard Edition would get early access to six Specializations if they played the game while connected to Xbox LIVE before November 20th. The e-mails for the Specialization codes have now been sent out, but there appears to be another reward for being part of the program. All of you gamers who have been a part of the "14-day Buy and Play" program will have limited time access for the "Crimson Map Pack" DLC.
Until Tuesday, December 18th at 9 AM GMT, those that participated in the "14-Day Buy and Play Program" will have complimentary full access. You will be able to download the DLC at no cost and play it until the cutoff time. This means that you will be able to unlock the achievements.
After December 18th, anyone who has not purchased the map pack will be unable to play any of the maps until you purchase the "Crimson Map Pack" DLC or the "War Games Map Pass". Those that have purchased the "Crimson Map Pack" or own the "War Games Map Pass" will be able to continue playing as normal. You can purchase the "Crimson Map Pack" now from here
for 800 MSP.
Thank you to all of the community members who sent this story in!