Just got some Assassins creed bundles and listing various fixes 😊
Wherever you extracted the file from Humble Bundle download links, navigate to that path. In Browse choose the end to be D2,D3,etc at each next prompt. This way you can continue installing the game.
Another one is in UbiSoft program -> go to black flag -> next click locate installed game -> choose the path (for example G:\blackflag\Assassin’s Creed IV Black Flag)