I'm surprised that no one knows what you are talking about. Well as of now you can't get a copy of it. I read a long time a go, I don't remember where, that she might release it, or parts of it, in the distant future. But as of now, there's nothing we can do to read it :/
The reson I joined this website is because if you do a search on forever dawn it comes up with a page discussing the details of the manuscript and someone was emailing copies out but it was from about a year ago. There are 4 different listing for it but the downloads don't exist any more (if they ever did) so this site is my best bet as far as I can see. Hope...
According to Meyer's website Meyer said that she really didn't think 'Twilight' would be a best seller, but it turned out it was. Before, she released 'Twilight' she wrote a manuscript for her sister.
Meyer relised that her audience was mostly young adults so, she didn't release 'Forever Dawn' because she believed it was to explicit.
But, don't fret, she basicly summed up 'Forever Dawn' in 'New Moon' and 'Eclipse" in a way that was more appropriate.
But, I have seen auctions for that book. (If you do try to buy it, the book would cost a whole lotta mulah)Good luck in your quest :)