Follow that tutorial
userChrome.css should look like:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#s3_translator-menuitem-_translate_selection { order: -1; }
For some security reasons S3.Translator does not work in 100% within reader mode so to make translating text easier (because of that one function works in reader mode...) Translate Selectedt Text menu item is at the top of that menu.