I just tried to install the addOn 'customFooter'. But after selecting and importing the addon-customFooter.xml file I got the error message 'Please enter a valid callback method.' In the install/upgrade instruction I found: "Please enter a valid callback method" will intermittently occur when the files have not yet finished copying before the XML begins to import. Just press back on your browser and click the Install & Upgrade button again." I have done this ten times, but always got the same error message.