In the Data Transformations section, first you will need to select the option to use 'Excel Import.'
You will then see a screen where you can select your file (by clicking on the binoculars).
This will defer to the first tab of your file if you are working with a file that has multiple tabs (such as .csv, .xls or .xlsx). Once you have selected your file, you will see the options to map where your fields in your file should go in the Data Transformation grid.
Selecting a 'File Field Name' allows you to assign a field from your File to the respective area of the Data Transformation grid. The Default allows you to assign the same value for all rows.
To watch a video demonstration of this process, click here.