To unlock the Native armour, you must first visit the Native hills and Ice Caves, correctly solve the puzzle associated with the Native Pillar there, and obtain the Native Totem. There are 7 in total to collect, and they can be found at the locations listed below.
If you look on the map, Native hills and Ice Caves are both identified by icons that look like caves. The co-ordinates for each are listed below.
The totems will all be marked on your map when you reach each location (you may have to activate a synchronize point to unlock the map in some cases; these will be marked already). Once you know where it is, there is usually an obvious path that you should follow to reach it.
Once you are there, you will have to solve a puzzle that involves lining up glowing lines with each other. This is not particularly challenging, but the video below shows you the correct combination for each totem.
Complete all 7 and you will unlock the following achievement.North Atlantic
Gros Morne - (483,-305)
Terra Nova - (797,480)
Fogo - (907,-67)River Valley
Aarushi - (923,-48)
Dekanawida - (500,-628)
Genessee - (360,-47)
Otetiani - (198,-430)
Finally, you will want to fast travel to Orenda in River Valley (224,-405) and unlock the Native Armour (272,-393).