When you say the function, do you mean the code, or are you using one of the formula approaches mentioned in the reference thread? If you're using the code, you can place it in the ThisWorkbook module as an On Open event. Numerous examples on the board. To get to this module, right click on the Excel icon to the left of "File" on the menu bar and choose view code, or open the VBE (ALT+F11) and select it from the left hand project explorer.
HTH
lenze