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.