OK, let me see if I can remember this. And I can't test it as I don't have Excel 2003 installed.
In Excel 2003, the WORKDAY function is part of the Analysis Tool Pak. To use it in VBA, do the following:
Open Excel; and select Tools/Add-Ins
Select: Analysis ToolPak - VBA (If that is not there, you will have to browse for it; try something like C:\Program Files\Microsoft Office\Office\Library or do a search)
Then open the VBA Editor.
Select Tools/References
Select the reference to atpvbaen.xls
You should now be able to use the Workday function directly in your VBA macro. So, in the macro, remove the "worksheetfunction." before the Workday, so it looks like:
dLastWDOM = WorkDay(DateSerial(Year(Date), Month(Date) + 1, 1), -1)