This achievement is completely story related and will be achieved in the first mission. When you first start the game you will wake up in a German u-boat. When you try to escape you end up being re captured and at the end of the cut scene you will have access to the power armour. Your achievement will pop a few minuets later when you need to use the power armour to get into a vent.
If you would like a video guide for what I have just explained I will leave mine below.