After you download a book, it should go to your media card. Go to your memory/media card and hit "menu" then "explore".
If you already have mobipocket installed, you will see a folder called "ebooks"
Ebooks must be in that folder for MP to read it.
What usually happens is that the ebook downloads to the very bottom of all those folders. So, scroll down to the bottom, hightlight the book and move it into the ebook folder.
You should be good to go.
Also, if you buy a book from MP, it will auto download into the ebook folder
Posted from my CrackBerry at wapforums.crackberry.com