Make sure you have firmware version 35a loaded on your jetbook (
ftp://custftp:FQSswyvGQ20So@ftp.ectaco.com). Version 35g is too buggy so don't bother with it... 35a is the one to go with until version 35k is released.
Then you need to download the epub into Adobe Digital Editions on your PC. Plug your jetbook in and you should be asked to authorize your jetbook by Adobe Digital Editions. After that drag the book to your jetbook and it should open fine.