How do I add an Excel Spreadsheet?

This is most likely an issue with the way your Web server is configured. It is still not configured to recognize Office 2007 file types (which are, actually, ZIP files – they just use different file extensions to identify which program should be used to open them).

Therefore, you can either save the spreadsheet in Office 2003 format (.xls), update the MIME types recognized by your server, or convert the spreadsheet to Google Docs and embed it on your site.