Both the ISODD and ISEVEN functions are part of the Analysis ToolPak add-in
for versions of Excel prior to Excel 2007.
If you're getting a #NAME? error that probably means you don't have the ATP
installed. Goto Tools>Add-Ins and select the Analysis ToolPak if it's in the
list of available add-ins. If it's not listed then you'll have to install it
from your original installation disc.
Or, you can try one of these formulas assuming your numbers are integers:
This will return either 0 or 1, 0 for even numbers and 1 for odd numbers.
=MOD(A1,2)
This will return either Odd or Even:
=IF(MOD(A1,2),"Odd","Even")