The script step you'll use is Perform Script. When you specify the options for that step, you'll see a list of available scripts. Notice at the top of the list it says current file. You can click there and select a different file. If you don't see the file you want, there's an option there to select a data source.
Let me know if there's something you don't understand, but really if your boss already knows this stuff he could show you in 2 minutes. Wouldn't that be a better use of your time?