To add / change the scrolling text you must work inside the html tab at the bottom of your editor. The code you will be working with is as follows:
<table cellspacing="0" cellpadding="0" width="780" border="0">
<marquee scrollamount="4" width="100%" bgcolor="#8A2930" :><font color="white"> Change this text to say to say whatever you want! </font></marquee</td>
Change the text to say whatever you want, shown above in blue. You can also change the color of the bar (highlighted in red). The scrolling text defaults to the text color you have selected inside your site settings but you can also assign the color in the code (highlighted in green).
You can add this scrolling marquee anywhere on the site. Just copy and paste the html.