Can't get global VBA macro to work globally

[fast_thing]

I'm trying to use a VBA macro to prefill the save dialogue box with the date, anytime ANY workbook is saved. The code (pasted below) is only working for the workbook containing the VBA code.

I've tried inserting the code into my Personal Macro Workbook, and also saving and loading it as an Add-In. I've tried putting the code into the ThisWorkbook module of each, and also the Module1 module. Nothing seems to work.

I'm new to VBA, and I'm sure I'm missing something simple (maybe a call to ActiveWorkbook somewhere). Can someone help? I'm using Excel 2011 on Mac OS X 10.6.6.

Here is the code:
Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
dateFilename = Format(Date, "YYYY.M.DD")
Application.Dialogs(xlDialogSaveAs).Show dateFilename
End Sub

Thanks!