This is an Imperial only quest. No matter if you are a Rebel on leave, you must be Imperial aligned and an Imperial Combatant in order for the quest starter to talk to you. To start the quest speak to Sgt. Ruwan Tokai inside the Massassi Temple on Yavin IV (-3038 -2960). He will tell you that he needs a piece of equipment delivered to a Warrant Officer. Should you say "Yes", a Death Star Fragment will be put in your inventory. You will also receive a waypoint to the Warrant Officer. Head there now.