The message you see when you open a file in Excel Services if a document library has not been set up as a Trusted File Location.
Excel Services requires that document libraries be set up as a trustworthy place to publish files to. Without taking this step, files saved to a library will not open in Excel Services. Instead, people will see this message: "You do not have permissions to open this file on Excel Services. Make sure that the file is in an Excel Services trusted location and that you have access to the file."
Make sure that any document library you want people to use with Excel Services is set up as a Trusted File Location. There are specific instructions for how to do this in the Quick Reference Card at the end of the course.