I just finished the mission on the moon of Palaven and I'm trying to romance Ashley. I'm about 2 and 1/2 hours into Mass Effect 3 and trying to find planet she is on. Where is she?
At that point in the story, Ashley is still in the Huerta Memorial Hospital on the Citadel. She is in a room in the on the right of the Inpatient Wing (the area past the main room with a store).
She will not join you until a later point of the story:
After the citadel is attacked by Cerberus, assuming you didn't kill her or send her off to the Crucible.
Finally, if you are trying to romance her, its recommended you buy her a gift at the store so you can give it to her later.