From
Gudy:
Ah, you're reading a Mobipocket book, then. Confusingly, bookmarks work differently for Mobipocket books than for most (or all?) other formats on the V3 clones, which I hope will be fixed soon.
For now, press 6 to get the bookmark page, press the big OK button to get the bookmark menu, from which pressing 1 will add a bookmark for the current page and pressing 2 will allow you to select a bookmark to delete.
To go to a previously set bookmark, I believe you need to press 6 to get the bookmark page, then press the number corresponding to the bookmark.