Once again, Desktop Manager 4.2.1 (4.2 SP1) is needed to transfer encrypted files. Quote from KB12999 noted above:
Only the the Media Manager for BlackBerry® Desktop Manager 4.2.1 will read encrypted files. Roxio® Media Manager for BlackBerry® smartphones uses USB Mass Storage and currently does not support encrypted media cards.Have you tried that yet?
If you have and it doesn't work, you may have done something to change your encryption key, in which case you will never be able to access the data (that's kind of the point!).