To get custom styles to display in the Styles list, edit the HTML editor instance. On the style tab, select the style sheet you wish to use, then select the classes that should be listed.
For the styles to display properly in the editor window when selected, you must attach the style sheet to the page you have applied HTML editor to.
Go to Window -> Style. In the style window, click the chain icon to link a new style sheet. Select the style sheet and click the OK button to attach it.