It is a function that can be turned on or off in the Maintenance > Data Maintenance > Module Customization. Select the module you want to change the default for and click edit. Go to the Attributes tab and look for the following attribute: Use HTML Editor. If it is not checked, then the HTML editor is turned off. To turn it on, select the attribute you want to change and click Edit. Then place a check in the box and click save/close.
Two things to keep in mind about the HTML editor especially #2:
1) it only works for system generated Document Templates (i.e. those Document Templates that have a (sys) in front of them. For non-system generated document templates (i.e. ones that populate into Microsoft Word), the html does not work.
2) If you create a document where the system generated document is initialed choosen and you use the HTML editor, but then change the document template to a non-system generated template, you will see HTML formatting in the new document template. You have to manually delete the HTML formatting. So for cases where my clients are using their own document templates I typically turn off the HTML editor for this reason so it will not cause any problems in the future.
Hope this helps,