Click on "redeem" on the page (by the 00 for armory box totals). This button will "check" to see how many you've earned, and add the total to the number on the left. You then click "open" to actually open one. It will not show any earned boxes or allow you to open them until you do this.
Yeah, it took me the better part of a day to figure that out myself. Call it a nice "feature" of the event - like still having some crew death on the 8th.