In order to import a spreadsheet into SNAP you must save it as a .CSV (comma separated value) file. When working in a spreadsheet go to "Save As" and select the .csv file type and save the file as that file type. Make sure the file is closed before importing it in SNAP. If it is open, the import will fail.
Included are links to some templates for importing different data types into topShelf via Snap. Be sure to save them as a .CSV file before importing them: