In "Options | Preferences", go to the "DHTML Editor" tab and enable "Use DHTML editor". You should then have both a Source and DHTML page for every document you open after that. Changes on either page are reflected in the other when you switch back and forth.
You won't see the tabs in any window that is already open before you set the Preference to use DHTML. There is a button to toggle between Edit and Browse mode: it is on the Format toolbar which is specific to DHTML editing. BTW, the little window at topish-right is a Windows Media Player OCX in an APL Form. Pretty cool, huh? :-)
When enabled, the standard PHE text editor and the DHTML editor both are shown on tabbed selector pages in each document window. You can switch back and forth between the two editors, and changes you make are updated in each window.
You can operate the DHTML editor page in Browse-mode only by checking the "Start in Browse mode" option in the Preferences dialog. You still will be able to switch to Edit mode in any DHTML window.
Hyperlinks are only active when the DHTML control is in Browse mode, so if you are using it to edit, you have to switch to Browse mode to activate links (on the View menu, uncheck DHTML Edit mode -- as I write this, it occurs to me I should have a toolbar button or keyboard shortcut to make this convenient.) You also should be aware that clicking on a link in the DHTML will open that page in your Web browser, not in the DHTML control (otherwise it would replace the document you are editing -- not what you want.) However, by saving changes you can go to the Web browser and Refresh to view and test. You also can open the saved document in a Web browser control window (contained within the editor) if that is more convenient (View | MS Web Browser).