The DEFINED pages are one of the few instances where ZC invokes raw html.
When you create content for a DEFINED page - by using the wysiwyg editor (like HTMLAREA) or the text input window, you are essentially adding stuff that gets stored as raw html.
Now remember... page elements in zencart are drawn from a variety of sources - with the basic page structure dependant on PHP. This is also true of defined pages, but the centre column content draws its elements from the html content of the files in:
includes\languages\english\html_includes\
So, it is quite reasonable to use an external HTML editor (dreamweaver) to create the content for a defined page - only that you do not use the whole page you create - just PARTS of it...
Which PARTS, and HOW ... ?
Essentially, you want the raw code of the elements you create in dreamweaver, so, once you have designed the page in dreamweaver, you need to invoke dreamweaver's "toggle to HTML Code" feature, to reveal to you the underlying html.
Then, you copy (to clipboard) parts that exist ONLY BETWEEN the <body> and </body> tags.
You do NOT copy the entire page's html, as this contains header stuff that your ZC will find confusing!
Then, in your Define Page editor screen of ZC, TOGGLE to html code. It's the <> icon in the menu.
Then <CTRL+V> paste the html you copied from dreamweaver into the input frame.
Bear in mind that only basic html is "safe" - if there are any obscure calls and jscript, for example, you may have problems.
Stick to basic HTML tags and you should be fine.