I also purchased the footer plugin and had the same problem
“The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature.
The solution of this issue is like you suggestet with the upload via ftp to the plugin folder, but:
1. you have to copy the directory too including the php-file
2. the plugin must be activated in the dashboard>installed plugin settings.
So, thank you very much for your help!
Kind regards, Gottfried