write click event for dynamic label

I would strongly recommend using jQuery, but then you should better use if for inserting the new label as well, to get a already jQuery-wrapped object. Please see:
http://api.jquery.com/category/manipulation/dom-insertion-around/[^],
http://api.jquery.com/category/manipulation/dom-insertion-inside/[^],
http://api.jquery.com/category/manipulation/dom-insertion-outside/[^].

Then the click event is written like this:

myLabel.click(function() {
    // some handler code here
    // by the way:
    var sender = $(this); // the jQuery wrapper of the sender object
                          // in your case the same label you clicked on
    // this way, you can use sender's properties in your handler
});

Please see: http://api.jquery.com/click[^].

If you need to learn jQuery (highly recommended), please see:
http://en.wikipedia.org/wiki/JQuery[^],
http://jquery.com[^],
http://learn.jquery.com[^],
http://learn.jquery.com/using-jquery-core[^],
http://learn.jquery.com/about-jquery/how-jquery-works[^] (start from here).

If you don't want to listen to a good advice and want to write it in Javascript by yourself from scratch, you can always add a handler by using

document.getElementById("myLabel").addEventListener("click", function(){ alert("Hello World!"); });

The sample above is a combination of samples shown here: http://www.w3schools.com/js/js_htmldom_eventlistener.asp[^].

—SA