You can select this option from the "Insert | HtmlHelp Control" menu.
Selecting this option will display a simple editor where you can add as much raw html as you like. When you click [OK] you will see a yellow <TAG> appear in the editor. This is your visual cue that there is a block of html code there. When you compile the html is added to your Topic and will appear as you expect.
To edit the <TAG> simply double-click it.
As an example of what you can do, select "Insert | HtmlHelp Control | <TAG>". In the editor type in "<b>I am bold</b> and I am not" and click [OK]. Now compile, you should see that this simple html has been added to your help file.
This is a HtmlHelp only feature and does not apply to WinHelp.