In the HTML site map interface are you able to see the items in the add option? Are you able to click on any of these or does it not allow you to?
When you are using the interface are you using only the mouse to navigate and make selections? If not please give this a try, I know that sometimes the interfaces can act strangely if you use the keyboard at all including the enter key.
Are you able to make other changes to the xml site map, like adding in a new item in that map?