I'm assuming you mean 'Function'.
If your User Defined Function is in a currently open workbook OR in your
'Personal.xls' workbook OR in an add-in you've created, you will be able to
use it by selecting...
Insert / Function
Highlight either 'ALL' or 'User Defined' in the "Function Category" window
Scroll down until you see your UDF in the "Function Name" window.
Highlight the UDF and select 'OK'
HTH,