Placing an "Execute" button in Excel...

Add a button from the Forms toolbar. Assign this macro to it.

Sub RecalcSheet()
activesheet.calculate
End Sub

If you want the workbook to recalculate you will need to use the following, but it will do all open workbooks.

Application.Calculate

Or loop through each worksheet

Sub RecalcSheet()
   dim shtTemp as worksheet

    for each shttemp in activeworkbook.worksheets
        shttemp.calculate
    next

End Sub