I know this is small but it is a bit confusing. The install.txt instructions says the following:
Installation is quite easy: copy the contents of the Classes folder to any location
in your application required.
The contents are the PHPExcel folder and the file PHPExcel.php
yet all the examples such as 13calculation.php located in Tests says the following:
/** PHPExcel */
/** PHPExcel_Calculation */
This seems to say you should have uploaded Classes and everything inside not just the content of Classes.