How to enable html source button on tinyMCE?

Add "code" into plugins list, then add "code" into button array

        tinyMCE.init({
            // General options
            mode: "textareas",
            plugins: "spellchecker,code",
            theme : "advanced",
            theme_advanced_buttons1 : "bold,italic,underline,separator,strikethrough,justifyleft,justifycenter,justifyright,justifyfull,bullist,numlist,undo,redo,link,unlink,code",
            theme_advanced_buttons2 : "code",
        });