For make it work you should activate your vote in Joomla (Content-Article Manager-Options and there you should activate Show Voting)
When you will activate this plugin you will see both of them (voting plugin of Joomla and Extra Vote).
The fast solution that I found during writing this review is to change names of .xml and .php files of Joomla plugin. So if it's Ok for you do it here: Plugins - System - Vote and I added "_" in the end of two files (vote.php and vote.xml).
Thank you very much for this plugin, it's really very nice :)