How to start the Omega DLC?

I had no trouble doing this on a save halfway through the game, with Leviathan halfway done to boot. You'll get an email from Aria, and then it should show up on the Citadel with a marker the same way Leviathan did. You may have to read this email first - I didn't check to see if showed up anyway on mine.

You will need to complete Aria's original missions from Mass Effect 3 (uniting the Blood Pack, Blue Suns, and Eclipse) before the Omega DLC missions become available.

You have to finish Priority Palaven first.

You will then get the invite email from Aria. When you head for the Citadel you will see two docking choices - one for the normal landing bay and a new one for another landing bay (the one that Aria gives in her email) Choose the new bay and you are good to go!

If you haven't gotten the blue suns, blood pack, and eclipse together it won't let you do the mission. Once you have done this then omega should come up on your email.