How can I call a user defined function from a custom menu?

I have created an add-in (xla) that contains several functions. These
functions work perfectly and show up under: Insert Function dialog box
under the category: User Defined

I am also able to create a custom menu by following the instructions
on the following awesome site:
http://www.ozgrid.com/VBA/custom-menus.htm

What I am unable to do is to call the functions from the custom menu.

Each UDF has its own module in the xla workbook and starts with Public
Function <function name(serial As String) As Integer

The OnAction line reads:

..OnAction = "<function name()"

Error message that I get: The macro '<function name()' cannot be
found

Can you please help me out what I do wrong and tell me what the
correct syntax for OnAction is?

I appreciate your help.