There is a Mega Event setting at Game Startup, under Game Settings:
Depending on what that is set to you may see few or even no Mega Events.
It also only applies to new games, so if it is currently set to "None", you wouldn't see any Mega Events until your next game.
Also, even without the DLC, you should have been getting the non-DLC Mega Events all this time. Perhaps you've had it set to None or Rare before this?
