i downloaded it but how to start it??

http://www.gamesas.com/topic/1427552-how-to-start-dragonborn/

Random encounter along a road. Took me two minutes.

you have to speak to the Greybeards for the first time. You don't need to do the horn quest, but to have to activate the quest. Once you have finished with the Greybeards, simply travel somewhere and the cultists will appear. I fast travelled to Riften and they appeared.

I've been on the road for awhile now and have been attacked by a dragon, a thief and a bosmer. No cultists.
And my character is level 42 and has done the Main Quest.

Try fast traveling between cities instead. They attacked me inside of riften.

ok i shall

they're walking, so maybe you went in the opposite direction. I did that with Hearthfire. It's only when i waited in the same spot for 24 did the courier appear.

ugh ive been playing for over half an hour and havent found any sign of a cultist yet. and i have all of the requirements met