Why do you think that she must be hypersexual to be diagnosed bipolar? The DSM-IV requires X out of Y in order to make the diagnosis, not X of X, or all of them. A 5 year old should not display hypersexuality, even if she had the worst case of bipolar disorder. To a large degree, people learn about sex from other people (though there is some innate understanding), and for a 5 year old to exhibit something that could be considered hypersexual would be more indicative of child abuse than anything.
Was it a doctor who said that is the missing part of the diagnosis or possibly a misunderstanding from what you read online? I can't imagine a doctor saying that, well I can, but the doctor I imagine is a pretty crappy one.