Just go to:
C:Program FilesAdobeAdobe Dreamweaver CS4configuration
Open file: Extensions.txt
edit line 16 and add THTML
Save and Close the File
Next, Open MMDocumentTypes.xml at C:Program FilesAdobeAdobe Dreamweaver CS4configurationDocumentTypes
Add THTML online 75 : winfileextension=”thtml,php,php3,php4,php5″ macfileextension=”thtml,php,php3,php4,php5″
Save, close and Restart Dreamweaver!
For more Info :- http://keithics.com/blog/article/opening-cake-thtml-file-with-dreaweaver-cs4/118/
Thanks
Jawed