You can enable the WebKit Inspector (which includes some tools for debugging Javascript) by quitting CSSEdit, running the following command in the Terminal, and then restarting CSSEdit:
defaults write com.macrabbit.CSSEdit WebKitDeveloperExtras -bool YES
To open the WebKit Inspector, right click in a preview window and choose "Inspect Element".
Original post:
Your best bet (if you can't get ahold of MacRabbit support) is likely going to be finding an alternate workflow. Although CSSEdit's override feature is a vast boon, it's certainly possible to develop CSS for an existing site using Firefox with Firebug and the Web Developer extension.
If you have access to a testing server (not the live environment) via FTP, you could also remotely edit the stylesheet using CSSEdit, and preview your changes in another browser (all Mac FTP programs that I know of offer an "Edit with..." option for remote files).
Good luck!