"He hasn't found me" was accepted. Without context both simple past "I was lost in the mountains and sent my husband a message before my battery died. He didn't find me but I managed to get home safely." compare "He hasn't found me. I'm still lost and I hear wolves howling..." ;-)
I feel like "did" and "has/have" are vaguely chosen as translations in this lesson.
If this set of lessons is in the Present Perfect (unspecified time before, up to, and including now), shouldn't this be "He has not found me"? "He did not find me" is in the Simple Past, I believe.
Verb tense source I'm using: http://www.englishpage.com/verbpage/simplepast.html