to build interest on excess cash/ cash revolver you need to "on" the "Enable iterative calculation" option (file -->Options--->Formulas). So,once the iterative calculation option is on, you can link the profit and loss line item (interest on cash or interest paid on cash revolver)with the balance sheet line item (excess cash or cash revolver). This will create a circular reference, which will help in calculating the interest received/interest paid once the numbers are inserted in the balance sheet.
For Dupont analysis we have just linked the required (selected) profit and loss and balance sheet line items and used the dupont formula to calculate the ROE (return on equity). The excel workbook is not password protect,you can check out how we have linked the line items and calculated the ROE.