[Baja]

It sounds like some of the required files weren't uploaded properly, most likely to the wrong folder. You want to copy everything inside the upload_this folder to your public_html directory. That means you will need to enter the upload_this folder and then "Select All" in your ftp client. It won't work properly if you simply drag and drop upload_this to public_html, thus creating a new upload_this directory on the site.

Be sure to upload all of the subfolders as well (admincp, images, and includes).