This should unlock for most people between the level of 13-15+. The achievement is unlocking at least 1 soldier from each unit (country), and there are a total of 12 units. This achievement can occasionally be glitchy if you do not fully complete games by quiting or disconnecting in a while that you have levelled. If you have had problems with it, just keep playing and it should eventually unlock.
You can also view your progress of Battlelog under the Soldiers tab, by looking in vertical columns.