Alright first off, have you tried re-reading "Waking Dreams"? (I know it sounds like a stupid question, but it is the lowest hanging fruit)
And now two things from the Wiki:
Quote:
If you prematurely pass through every chapter of the Black Book: Waking Dreams before Frea tells you to avenge her father, visiting Apocrypha may not prompt Miraak or the dragons to attack.
Quote:
The quest may not have been entered into the journal, despite completing the prior quest. Use the console command setstage dlc2mq06 200 then walk around the locked gate to trigger the next objective and [Sahrotaar] should arrive.
So my suggestion is to read "Waking Dreams" and enter Apocrypha, walk as far backwards as you can, then type in console command dlc2mq06 200 and proceed through the area.