Beat Chapter 4 first
The first wave of downloadable content (DLC) has hit Fire Emblem: Awakening, and to buy it, you need to do a little bit more than just load up the eShop.
As previously mentioned in my tips guide for the game, you need to progress past Chapter 4 to unlock the Outrealm Gate towards the southern end of the map (pictured). Once you're there, just enter it, and select the purchase maps option.
Right now, there's three DLC options available: Champions of Yore 1 ($2.50), Champions of Yore 2 ($2.50), and the Champions Pack ($4.00). The Champions Pack is a bundle, and will also include Champions of Yore 3 when it comes out. Champions of Yore 1 is free until March 6th.
Each DLC is around 30 blocks each, so space wise it isn't going to get crazy. As is the case with Japan's Fire Emblem DLC, the packs will feature their own story, their own battle map, and a recruitable character (Marth and Roy for Yore 1 and 2 respectively).
One major perk the add-ons have is that every one of them is repeatable -- so essentially, it's an easy, controlled, guaranteed way to grind should you need it.