Your Excel file may contain special formatting—borders, colors, special characters, etc—that are causing an issue with the upload.
The system doesn't accept any special formatting - so make sure your file is squeaky clean. If you clear the formatting on your XLS document, that should take care of it. Another good trick is to try saving your file as a CSV, and uploading again.
If it still won't go through, let us know. We'll take a look!