Uite aici un exemplu, insa nu are butonul de close.

Pune te rog http :// in codul <script type="text/javascript" src="code.jquery.com/jquery-1.7.1.min.js"></script>

Cod HTML:
<style type="text/css">

.popup{
    border: 4px solid #6b6a63;
    width: 800px;
    border-radius :12px;
    margin : auto;
    padding : 20px;
    position:fixed;
	
}

</style>

<div id="popup" class="popup">
some lengthy text

</div>

<script type="text/javascript" src="code.jquery.com/jquery-1.7.1.min.js"></script>
<script>
$(document).ready(function(){
    var popup_height = document.getElementById('popup').offsetHeight;
    var popup_width = document.getElementById('popup').offsetWidth;
    $(".popup").css('top',(($(window).height()-popup_height)/1));
    $(".popup").css('left',(($(window).width()-popup_width)/2));
});
</script>