Hi @Pjtraveler, something looks broken with your paths.
The correct path to the installer should be /backlight/installer, but for some reason there is no backlight in the URL. If you look at the directory layout in your FTP program, do you have a backlight directory directly under the top of your site? (e.g. a structure like public_html/backlight or www/backlight where public_html or www represents the top level folder of your web documents).
Edit: this assumes that you are setting up a new Backlight site. If not then let us know:
if you had extracted the installer files locally, you should already have a structure that includes backlight as a directory. You need to upload everything unzipped as-is, so that the files and file structure in your www or public_html directory match the structure extracted from the zip file.