I haven't been able to actually play Mass Effect 3 too much yet and was wondering how to start the From Ashes DLC so when I jump back on I can. I've never actually been able to start DLCs on the the previous game. It would help me if someone would point out when and where these extra events occur throughout the game.
Look for the mission called Priority: Eden Prime. Assuming you have the DLC installed, this should be available to you in the game as soon as you board the Normandy and can use your Galaxy Map.
It is, in fact, the first thing I did upon having a choice about where to go.
As a heads up, there is a bonus mission on that planet called Eden Prime: Resistance Movement. Make sure you've done everything possible on that before completing the main DLC mission, as it appears you can not return to Eden Prime later if you missed anything.