Download and enable the FileBrowser plugin which is inside ckeditor/plugins/
:
var browser_plugin = 'filebrowser_upload';
var plugin_path = 'http://example.com/ckeditor/plugins/'
CKEDITOR.plugins.addExternal(browser_plugin, plugin_path + browser_plugin + '/');
var ckeditor = CKEDITOR.replace('editor', {
extraPlugins: 'filebrowser_upload',
extraAllowedContent: 'img[src,alt,width,height]',
toolbarGroups: [
{ name: 'basicstyles', groups: ['basicstyles', 'cleanup', 'colors'] },
{ name: 'tools', groups: ['tools'] },
{ name: 'upload', groups: ['filebrowser'] }
]
});