Ran into an issue with the Magento Connector where sometimes the fields which show in the dropdown, and sometimes they won't.  Additionally, you can process the data successfully, and after that, the fields WILL show in the dropdown.

1. Open already created Magento Connector.

2. See that fields are already mapped.

3. Trying to edit the fields, will cause all of the dropdowns to become empty.

4. Hitting cancel will make sure the fields don't get wiped out.

5. You can then process the data, and it will process successfully.

6. When you try to edit the fields now, they have no re-appeared, and you can map or un-map them.