Hey, sorry, my bad. Game was stuck in simplified mode.
New version (shadows_v10_bugfix1) should fix this. You should now be able to select any of the flashing purple nobles (in standard game mode) and hit "enthrall noble" from the power screen.
Or, if you want to start higher up, your agents can try to infiltrate a duke/king's location and then enthrall them using agents. Harder, but gets you a much higher rank noble.