I've noticed that sometimes after you've created a new attribute that it doesn't show in the Add Definitions screen. Doing a Performance->Clear Cache also doesn't do anything. What you have to do is go back into the Attribute (Store Config -> Attributes) and Edit the missing Attribute and just click on Submit again. When you go back to the Add Definitions you'll see the Attribute now listed.

It seems something to do with the Label field when creating an attribute. When you initially create an Attribute you only have to enter the Name field and the Label field is automatically populated by Attribute Module/Ubercart/Drupal when the submit button is clicked. If you let Attribute Module/Ubercart/Drupal auto populate the Label field then it won't show up in the CANO Add Definitions screen until you go back and click submit again. However, if, when creating an attribute, you make sure that Label has some text in before you click the Submit button then CANO will see it straight away..