you have to do all the missions especially any L missions and i think a yogi an Reggie as well as a cut scene with Bhadra
the one that unlocks willis is usually the chemistry mission
after that you should see a W on your in game map
Amita Mission: Advanced Chemistry
Sabal Mission: Basic Chemistry
if you need a mission list look here http://www.ign.com/wikis/far-cry-4/Campaign_Missions
again it depends on whom you pick to lead the golden path as to your mission order
the mission order is just slightly different
either way you still have to do all the missions like the one for Norre after you do the Amita mission the sleeping saints
Amita and sabal have similar missions but your objective may differ , this changes the game Slightly as to mission order
example is the city of pain mission (the norre mission )
if amita leads you have to take pictures of people being tortured
is sabal does you need to rescue people
the Willis mission will pop up usually after you do a few L missions and one for the golden path .. the chemistry one i referenced
after that you will get a call from Willis and a W will show up on your map
you have to do all 3 willis missions till you get a chance to go north
you have to do the one for Norre (city of Pain) first i think and like i said you have quite a few missions till you get the willis ones .. you get those at about 50 % of the game
the last willis mission ENDS ACT 2 its called Death from above .. after that mission you begin ACT 3 , you'll have one more mission then you get a Key to the North , and only after that can you get to into Northern Kyrat