I get the same error messages as the writer of the original message.
I removed the extension from extension Manager, downloaded the extension again and re-installed, after removing the MacDat cache file first as you suggested.
Using Dreamweaver CS4 on a Mac with OS 10.5
I even tried creating a new page rather than modify a menu on one of my existing pages, just in case there was problem with the page itself. This is where the problem started for me. I have used this extension before with no problem and had two pages on my website working. I have not tried to modify another menu created with Menuwriter on another page just in case I run into the same problem.