The first row in your spreadsheet must define the column names. Those will be mapped into the fields of your list in myphoner.
If your spreadsheet has a heading or notes on the first row(s), before the column names and the lead data, delete those row(s), so the column names is on the very first row in your spreadsheet.
Here's an example of a well formed spreadsheet (although rather small), ready for import to myphoner:
Still having trouble? Try exporting it as .csv (comma separated values) format.
Screencast explaining how to convert an excel file to CSV
You can convert an Excel worksheet to a text file by using the Save As command.
Click the Microsoft Office Button Button image, and then click Save As. The Save As dialog box appears.
In the Save as type box, choose the text file format for the worksheet. For example, click Text (Tab delimited) or CSV (Comma delimited).
You can find more details here:
Pro Tip: If you haven't already, give OpenOffice a try. It's a great alternative to Excel - it reads excel files and has better support for both reading and writing csv-files, it's completely free and comes both for Mac OSX, Windows and Linux.