Forum Discussion
Adding new Excel Functions
If the function is intended for use in a specific workbook, make sure that you create the code for the function in a standard module in that workbook. A standard module is the type of module that you create by selecting Insert > Module in the Visual Basic Editor.
It won't work if you create the code in the code module of a worksheet or in the ThisWorkbook module.
Make sure that you save the workbook as a macro-enabled workbook (*.xlsm or *.xlsb); if you save it as a .xlsx workbook, all VBA code will be lost.
If the function should be available in all open workbooks, you must create it in your personal macro workbook PERSONAL.XLSB. If you don't have this yet, you must create it by recording a macro in your Personal Macro Workbook:
After that, you can remove the dummy macro that you recorded, and create your own functions.
To refer to a function in your personal macro workbook in a formula, prefix it with PERSONAL.XLSB. For example:
=PERSONAL.XLSB!MyFunction()
or
=PERSONAL.XLSB!MyFunction(A1,A2)
When you quit Excel, it will prompt you to save changes in PERSONAL.XLSB. Answer Yes!
Hi,
I'm on Mac OS and still a bit stuck. I'm working with a UDF, not a macro. I can create the function and use it with my current workbook, but it only shows up in my list of functions with the PERSONAL.XLSB! prefix. I would love to not have to type all of that every time and I would also like to be able to reference it from different workbooks. Is this possible on Mac OS?
(Should I create a new post/question for this?)
Thanks in advance 
If you want to be able to use the UDF without a prefix such as PERSONAL.XLSB! you have to store it in an Excel add-in (*.xlam). See for example Method 3. Create an Excel add-in file