g... i guess that's because phpchat is more PHP than plain HTML. you should use a text-editor like ultra edit or switch to the source code view in dreamweaver.
Anyway... most likely you won't be able to edit the chat templates like a normal HTML page. in application like this you have to look out for variables (constants) that define the look and color of the app. Also watch for HTML-code snipplets embedded in the PHP code.
I customized phpMyChat and it was quite a bit of searching until i found the code that i needed to change.