I bought the From Ashes DLC ages ago and have played it through before, but haven't played ME3 for ages and decided to start a new play-through. I started the game without realising that the DLC was not installed, so I installed it halfway through the game. Does that matter? Is there anything I need to do to trigger it, other than Priority Mars which I did ages ago?
I Googled this a lot to no avail and I've now figured it out, so I thought I'd share in case anyone else gets stuck. Basically, problem 1 was that my XBox 1 said "installed" after I selected each individual add-on but, in reality, that just started the install process. It didn't actually finish until about 20 minutes later. No joy after going back to Eden Prime so I saved the game, went back to the main menu so that it could check downloadable content again, and resumed the game.
I was still orbiting Eden Prime while saving, but the mission has now appeared and, therefore, I assume all the others have too if people are just searching for general ME3 DLC advice and wind up here.