Sounds like there is an extension installed that is not supported in Dreamweaver CS5.
can you send screen shots showing your extension manager so I can see what is installed and the versions.
The quickest way to get back up and running would be to rename the configuration folder.
Close Dreamweaver, then locate the configuration folder at:
/Users/<Your User Name>/Library/Application Support/Adobe/Dreamweaver CS5/en_US
where <Your User Name> is the user name you log into your computer with. and rename it.