The really isn't any additional dialog regarding talking to them about Dagoth Ur after he tells you to convince everyone that the Empire and the Tribunal are the enemy, NOT him (and I have examined this mod thoroughly in the CS. Yes it's disappointing). You just go through the normal procedure to become Hortator and Nerevarine. After each House names you Hortator/Nerevarine, their dialog regarding Dagoth Ur will change.
After you are named Hortator and Nerevarine with each House/Tribe, they will say this:
For generations we have been taught that Dagoth Ur is the enemy. Now our leaders say that Dagoth Ur is to be our ally against the Tribunal and the Empire. I'm not sure about this myself, but I have faith in the judgements of our leaders.
They will then act like you have already told them everything about the Tribunal stealing the Heart's power. One Redoran councilor will end up saying something like, "I expect deception, but not from my gods."