Re: Mass Effect 3 (Citadel DLC)

The mission doesn't activate until after you have completed the Battle of the Citadel priority mission in the main story line. (There are character-specific content that will unlock at various different points, afterwards, once their plot points in the ME3 story have been resolved)  Afterwards, you will receive an e-mail at your private terminal and reading it will start the "Shore Leave" objective and the Citadel DLC missions.