You might have failed the Hanar mission. If the quest is still in the log, you should be able to complete the quest and then have the option of sending Kasumi to the fleet. Completing "Priority" missions advances the timeline and the attack on the Citadel makes some of the quests uncompletable.

Actually, once you complete the Hanar mission *SPOILER* Kasumi "dies" because of an explosion. I say "dies" because her body falls unconscious and then her stealth devise kicks in and becomes invisible. I do not know if there is a different outcome to this event, but this is what I've got so far.

Kasumi travels to the Crucible to help build it.

*SPOILER* You have to complete the hangar diplomat mission before the citadel Coup attempt. Kasumi will appear to die but then after the specter leaves you tell her to come out. You can convince her to join the crucible project. She becomes a war asset. Other than a few emails and a conversation before the end of the game, you don't see her again