If you want the giants companions you need to have the culture Free Folk and go to a cave in the lands of always winter, but if you want troop giants you will need to capture them when fighting mace rayder(somthing like that), and recruit them via the recruit prisoners button

What you say is true Sexybeast. However, supposed to also be able to recruit giants from Frostfangs camp if both Free Folk in heritage and have enough renown. Do not think this is working right in version 6.2. Sent Produno a message regarding this as part of organized bug report. Hopefully it will be working correctly in version 7.0.

I know this is late but I'll still answer for those who are in need of an answer in the future.

There's 4 ways to recruit Giants in this mod ;

1. Capture then recruit them.

2. Frostfang camp. 3 requirements are needed;
-Enough Renown
-Be the lord of Frostfire Camp
-Be a wildling(race : freefolk)

3. Raid the Giant cave and there will be Giants there obviously, you need ti defeat all of them then after that 2 Giants will join as a companion(an Archer Giant and an Infantry Giant)
-Location, west of Thenn

4. Just walk around the land beyond the wall.
If conditions are met giants will join(Dunno the actual condition but you need be a wildling too)