Marketpress product upload causes 403 error...

Marketpress product upload causes 403 error. When creating a new product, I go to the area: Product Download > file URL (form field) > Upload File.

I do not see a 'browse' button, and when I add the URL of an image file, I get a 403 error. This error asks me to insure that I have an index file (which, of course I do). I am then directed to reset my site permissions, which I did – but no joy.

Any suggestions?