Before adjusting the cable ensure that the derailleur is throwing over enough, this is the thing with two small cogs that throws the chain off one gear and onto the next one, there are two screws that controls the limit so that you can't throw the chain off, try adjusting the one nearest the gear you can't get.
Peter
October 2007
adjust the leangth of the cable it worked for me
joey
June 2007
Not sure about this as i still haven't gotten up to cassettes, but search on sheldon brown; he has tons of info on the web on bike repair, and their shop, near boston sounds like it has great guys there.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix bicycles.