From FCKeditor Docs

LinkBrowserURL

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

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

or for any custom implementation:

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