So, looking through all the agenda cards, I cannot find how to get the ISB side mission unlocked so you can get the tactical advance reward card. Anyone know what agenda deck has this? Or how to unlock this side mission? I am wondering if I misplaced those card.....games becoming a lot to manage.

Cautious Approach and Tactical Advance go together. And the instructions in how to include Into the Unknown in the Besbin Gambit minicampaign is in the ISB Infiltrators rulesheet.