You will need to create it outside of SiteBuilder in a photo-editing program. There are many out there and the costs vary, but there is a nice FREE program you can download at: http://www.getpaint.net/
You will be creating a .jpg or .gif file which is a picture of whatever text you are creating. You then upload to SiteBuilder and place it on the page where you want it.
Also if you happen to have MS Publisher, or Powerpoint already on your computer you can do the same thing. After you create the diagonal text, save it as an image file and import the image into SiteBuilder.