HTML Editor 3 is not compatible with DW CS6, it also contains an outdated version of the CKEditor files that may not be supported in your browser.
I recommend upgrading to Data Bridge which uses a more up to date CK Editor version.
or you can manually upgrade the CK Editor files.
in the webassist/ folder, rename the ckeditor folder to ckeditor_bak
On this page:
click the button to launch CK Builder. Select to use the Full Preset, and the Kama Skin.
Copy the downloaded CKEditor folder to the webassist folder in your site.
from the ckeditor_back folder, copy the following three files:
and paste them in the new ckeditor folder.
you will need to clear the browsers cache to see the updates take effect.