In the Mortgage Maker (SS) template included with the ClarisWorks Home Pack, cell H19 is meant to be an entry cell for "Additional Principle Payment Per Year." This cell incorrectly contains a formula and is protected.
To fix this, click on cell H19, choose Unprotect Cells from the Options menu, then type in the desired number. If no additional payment is desired, enter a 0 (zero). Remember to choose Calculate Now to recalculate the spreadsheet.