David – Please do the following checks
1. If you are import new data, then have you selected “As a new project” in “Import Mode” step? If you are appending data to the existing plan, have you selected “Append data to the active project”? If you want to merge the data in the file with schedule data, then have you selected “Merge the data into the active project”?
2. Assuming, you want to import new data then in Mapping Options step, have you selected “Tasks” / “Resources” / “Assignments” check boxes and checked “Import includes headers”?
3. In the “Task Mapping” step (assuming you have checked Tasks in the previous step), selecting “Source worksheet name” will display the column names of the selected sheet in “Excel field” column