After selecting the Use existing Tables and Queries, you will now be prompted to choose the fields to use in the subform. Now make a selection for the table or query from the drop-down list.
From the left panel, select each field that you want to keep on the subform, then click the little > button to move it across to the right pane.
You can choose fields from more than one table or query. After completing the first one, select another table/query from the drop-down. After completing all this, just click to the Next option.