Thanks for the quick response Mr. Wang Bin
I'm sorry, maybe I did not understand. I tried for a week.
1) I tried to paste the code in the header.php theme of twenty eleven
2) I tried to install other themes
In any case, the plugin only works in the back end, the switch does not appear in the front-end
3) The problem may be the WordPress version 3.4.1. ?
The trouble is I'm not a genius of php, could you tell me more clearly the code, the file and the place to paste it?
thanks again