I also have a similar problem upgrading from v1.0.4 to v2.0.1 but on an XP machine. I installed the new extension and then opened DW CS4. OK so far, but when I tried to open an existing Menu with the new extension - I got an error message that DW has encounterd an error and needs to quit!
This looks like a serious problem - unless there are some special instructions for un-installing the old version and then installing the new one.
Help!