The Field column on the left shows the name of the fields as they appear in the table of the Database:

Select one of the field names and click on the Change… button, this will open the Insert Variable or Expression dialog box in which you can select (or create) the name of the variable you want to contain the value of the field.

In the illustration above, product_id is the name of the variable in Opus that will store the value of the field Prod_id in the current record.