From FCKeditor Docs

FlashBrowserURL

Defines the URL to load when clicking the "Browse Server" button in the flash dialog. It can point to the default File Browser:

FCKConfig.FlashBrowserURL = FCKConfig.BasePath 
+ 'filemanager/browser/default/browser.html?Connector=../../connectors/' + _FileBrowserLanguage + '/connector.' + _FileBrowserExtension ;

or for any custom implementation:

FCKConfig.FlashBrowserURL = '/myfilebrowserpath/browser.php' ;