The default TW editor is a HTML text-area. By default they do not allow to insert tabs, since HTML usually does not show it anyway.
I did create a short HowTo, so users can create their own tab-button including the tab-shortcut to get it going.
See: How to Use the TAB Button in the Editor which I just created.
Be aware, that the TAB character cannot be used to indent paragraphs in rendered HTML output
Have fun!
mario