hi Peter, sorry to hear you are having trouble. i can't help you much but just one clue, i know that mobipocket dictionaries must be formatted in a specific way to work, otherwise they are just seen as regular books. i am not sure the version you found here is correctly formatted as a dictionary. try this one :
http://www.ebooksgratuits.com/details.php?book=1609 which i am relatively sure *is* formatted correctly, as a test.
if that still doesn't work, maybe try posting your question in the mobipocket forum. good luck ! i hope you solve the problem.