edit the HTML Editor instance, click the edit icon next to the Preset list.
On the styles tab, click the edit icon to edit the styles listed in the styles menu
In the top section of the screen, it will show the style names.
The bottom section allows you to specify the attributes applied when the style is selted.
click the plus button and you select Tag Attribute. For Setting, enter "class" and for the value enter the name of the class you would like to apply