How to read excel files using objective-C in iphone?

I think the short answer is:  you can't !

http://social.msdn.microsoft.com/Forums/en/innovateonoffice/thread/67495a8e-0948-4807-91d0-8ec28c1fda8a

The Microsoft Office SDK uses COM (and .Net) which is only available on Windows and works well from all the scripting languages.  An IIS web server could read the spreadsheet, so you could use a web service to deliver data to your iPhone.