How to enable/disable next mission?

How can it be done? When I'm using Trigger "Game - Enable/Disable Mission" then I can't put my next mission name in it, there are only oringal campaign mission names in the list.

You must use the Enable/Disable Custom Campaign Mission action and give it the number of the mission you're trying to enable. (In the campaign screen, the map at the top is number one, the next one down is number two, etc.)