Do you actually want to write to an Excel workbook (I'm assuming Excel), or simply to a text file that's formatted in such a way that you can read it in using a spreadheet program? If you actually want to write to an Excel workbook then you need to use ActiveX. An example ships with LabVIEW that shows you how to do this. Open the Example Finder and search for "excel". You can also peruse the Excel thread.
The alternative, which is much easier, is to simply write out a simple text file using the Write to Spreadsheet File VI. This creates a delimited text file (like a .csv file) that can be read in using a spreadsheet program.
