The Main Quest pops up every five levels (...35/40/45) but to get the last one, you also have to complete all the Story Quests in Coldharbour. There's a list of those here:
Coldharbour: Army of Meridia Story Quests

The Coldharbour story is in four main sections;

• The first is arriving in the city, and starting the Army of Meridia quest to go off and look for allies around Coldharbour (you need to do the Side Quests to get all the allies).

• It is while you are travelling to the locations in the west and east to do the second and third sections that you will come across the Side Quests needed to gain allies.
• The fourth section, after you have recruited most of your allies and completed the Army of Meridia umbrella quest, is breaking into the northern section of the zone and reaching the far north where the Planar Vortex lies.

Mages Guild quests are dependent on your Mages Guild rank, so you need to be Rank 1 to get the first quest, and so on up to Rank 5 to get the fifth quest. The Mad God's Bargain is the Rank 5 quest, so once you hit that rank with the Guild the quest should start when you next go to a Mages Guild hall.