How to make an existing content type a product in Ubercart 3 and Drupal 7?
Have you simply tried to put the machine name of your existing content type in Class ID, and its human name in NAME ?

I've had such problem 3 years ago, and I solved it so. More, for tests purpose I've just created a foo content type. Then I added the Class ID foo, and it has been mapped with foo content type.