I managed to figure this out yesterday - That blue button is indeed important. But here is the twist: you must have that toggle "on" when the event fires that enables those markers. So turn it on before July 1861. If it is off when the event fires, then they never show up and no amount of toggling ...

Playing the Blue and Gray campaign as the Union player, I sometimes find the markers that are supposed to show up in September 1861 to show where to place naval units to prevent/discourage European intervention never show up on screen. I think the "effect" remains working as long as vessels are ...