Where is core/config.js. ??

need to activate the stylesheetparser plugin

Stylesheet Parser Plugin

CKEditor 3.6 introduced a simplified method of customizing the styles for the document created in CKEditor and populating the Styles drop-down list with style definitions added in an external CSS stylesheet file. The Stylesheet Parser plugin (stylesheetparser) lets you use your existing CSS styles without the need to define the styles specifically for CKEditor in the format presented above.

To use the new style definition method you need to activate the stylesheetparser plugin for your CKEditor instances by using the extraPlugins configuration setting:

config.extraPlugins = 'stylesheetparser';