You will need to insert the custom function into your workbook as follows

1. Open your workbook
2. Press ALT-F11 to open the VB Editor
3. Right-click on the VBAProject for your workbook in the Project Explorer (left hand panel by default) and click Insert -> Module
4. Paste the custom function code into the main panel

Note that the function will only be available within the workbook to which you copy the code to.