Hi Joce,

I'am actually testing my 2.5 extension on a brand new Joomla! 3.0beta2.

The MVC structure and native code generate with j-cook match with the 3.0 API.
The main thing is that the DS contant is used everywhere in the files of the component. DS contant was still supported in joomla! 2.5 but as been removed in 3.0.

Source : Potential backward compatibility issues in Joomla 3.0

Solution is to add the following code to use a 2.5 extension under 3.0 :
if(!defined('DS')){
define('DS',DIRECTORY_SEPARATOR);
}

Marc