QTranslate extension

Hi everybody,
When I read how to use this plugin, I can read this:
“Simply add the class “multilanguage-input” to all the elements which you want to get multilanguage (input-text, textarea). It will make the elements multilanguage, so the information will be saved using the qtranslate structure <!–:en–>Content EN<!–:–><!–:es–>Contenido ES<!–:–>“
My question is: where have I to add this code?

Thanks