The Script Editor is not part of the default installation. The first time you activate this feature it initiates auto-installation, so keep your Word 2003 or Office 2003 CD-ROM handy.
As far as the HTML source code is concerned, you can search and edit the HTML source code from within the Script Editor as you want. To save your changes, choose File, Exit from the menu.
Directly modifying the HTML source from within the Script Editor is not recommended unless you are an experienced programmer or Web developer, or if you want to learn more about HTML by experimenting on prebuilt code.