Try this:
http://blog.contextures.com/archives/2013/01/24/show-the-file-open-window-in-excel-2013/
I can't believe someone thought making the backstage the default for something as simple as opening a file was a good idea. Way to complicate an otherwise simple process.
ETA cliffs notes as per comment below:
Click the File tab, and then click Options. Click the Save category, and click "Don't show the Backstage when opening or saving files" Click OK to save the settings