Correct. You can run BlackChat without MDS nowadays today. But you're on a BES, so that's the catch-22. See if you can ask your BES administrator if they can turn on MDS.
If not, then another hope might be had by configuring Options->TCP to the appropriate APN setting: http://www.blackberryforums.com/viewtopic.php?p=20472
However, if this doesn't work even after rebooting your BlackBerry, you might be stuck: Unfortunately, there's a way for BES administrators to block you from running such software.