You need at least 5 dark points for the choice to be available
If you really don't want to affect your current path (i.e., don't wanna go back to earn dark points by being a 'bad boy', etc.) and don't want to start a new game, but still really wanna see the scene, you can d/l the
URM mod and increase
x_dark_points to w/e you need it to. If you choose to continue your save with the cheated points, however, keep in mind you can potentially fuck something up in future content I think (for example, you might get too many active paths that run into each other and cause some kind of conflict); it might not, I'm not completely sure, just keep it in mind as a potential risk, maybe keep this save around so you can go back.
If you already had at least 5 dark points and it still won't work, you don't actually have patch enabled and will have to properly activate it.