When you click on your desired selection from the menu, a "Edit WordArt Text" menu should appear with the words "Your text here" already inserted in the "Text:" area of the menu. Are you not getting that menu at all?

If you do have that menu, you simply type over to text already there (as mentioned above) and click "OK". Or, simply click "OK" and the text is inserted into your document then you can type over it/modify the text to the text desired.