From Help on CUMIPMT.........
If this function is not available, and returns the #NAME? error, install and
load the Analysis ToolPak add-in.
Then it shows how if you don't know how.
Have a look.
If ATP is not avilable in the Tools>Add-ins dialog you may have to install it
from Add/Remove>Change in Control Panel.
Have your CD handy.
Gord Dibben Excel MVP