If this doesn't unlock straight away for you after you get your penultimate achievement try joining a multiplayer match. It unlocked for me as soon as I loaded into a multiplayer match.
I can't claim this is an ultimate fix, just letting you know what worked for me and hopefully it might work for you too.
Edit: It may also unlock by loading up a single player mission again after getting all the other achievements. This may be the better tip for the future as I'm sure the multiplayer will be dead again after the free play weekend finishes. Thanks to Ale386.