If you encounter 404 error when you connect to Magento Connnect Manager (System-> Magento Connect Manager), it is probably due to your folder permission not set up correctly.
Fortunately, there is a quick fix for this.
Download the magento clean up tool (a php file) and upload to your store root folder.
Once done, browse to the clean up tool (http://www.yourstore.com/magento-cleanup.php) and you will see a list of messages that it has reset the folder permission.