There is an easier method. using the GUI of Office 2007.
highlight the cell you want the 'total' to be in.
--->click on the 'E' sum button (usually on the 'home' ribbon, under the 'editing' section)
--->go to the other sheet, highlight the Cells you wish to Add/sum,
--->Press Enter.....your done. (do not navigate anywhere else before you press enter as this will change the formula)
Explanation:
=SUM(otherSheet!cellRange1:cellRange2)
Example formula:
=SUM(SheetTwo!D24:D26) this will add cells d24 through d26 on another sheet called: SheetTwo.
Example of adding cells from 2 different sheets:
=SUM(SheetTwo!D24)+(SheetThree!D10)