When I add a datasource as a CVS datasource and open the data file, it by defaults opens in Excel. Is there an IDE setting that I need to update or change in order to get it open in Notepad++ instead of Excel?
try right-click and then Open with in Navigator View.
In your case Excel is probably system-wide tool to open CVS files, so you have to change it in MS Windows.
Use internet to find answer how to: How to Change File Associations in Windows