You can use something like
Demo
$(window).on('click', '.mybutton' ,function() {
alert($(this).parent().parent().find('.nr').text());
//Or you can use, which is even better
//alert($(this).parent().siblings('.nr').text());
});
Here, the selector is pretty simple, we are first binding click event on the button, and onclick we select the button element parent i.e td and we select the parent of td i.e tr and later we find an element with a class of .nr
You can also write td.nr instead of just .nr to be more specific.
Your rows are being dynamically added, in order for your click listener to work you will have to delegate the events
Credits to @Patsy Issa for suggesting .siblings()