Hi, James. Thanks for responding. Here's what I discovered about using ImportWP PRO with the Toolset Types plugin.
My custom field names need to be prepended with wpcf- , so for example, a custom field named "location" has to be entered as "wpcf-location". During the ImportWP PRO setup, I had to Enable Text Field and enter these names manually since they did not appear in the dropdown.
One thing that confused me was that the ImportWP PRO setup interface made it look like I would be creating a new field/row instead of posting to an existing custom field. Perhaps this could be clarified with a tooltip, or some other text that says "Add row or post to an existing row".