For those with the same problem : I´m loading less.js as an outside plugin and after that loading less_helper.js wich is a script to load the main .less file :
less_helper.js :
var link = document.createElement('link');
link.rel = "stylesheet";
link.type = "text/less";
link.href = "<filepath>.less"
less.sheets.push(link);
less.refresh();
Reference : http://stackoverflow.com/a/14231762
Thanks