JQuery :: How To Add Calendar To Textbox

I am trying to embed a calendar into my customers pages - the JS, CSS and jQuery are held on my server, so to allow my customers to just have to add one line of code, the link I give them, links to a JS file, which then dynamically loads the other files required, then adds a textbox, dropdown and button to the BOM, using document.write. However, to add the jQuery calendar to the textbox, I have to be sure jQuery has loaded. If it hasn't, by the time the page script hits the $( to check for jQuery, if it hasn't loaded, all of the jQuery is ignored. How can I get the script to stop and wait (without having the customer having to press a button)?

So I give my customers this one line:
<script src="[URL]" language="javascript" type="text/javascript"></script>

The mt.js file contains...
//JS to load files
function loadjscssfile(filename, filetype) {
if (filetype == "js") { //if filename is a external JavaScript file
var fileref = document.createElement('script')
fileref.setAttribute("type", "text/javascript")
fileref.setAttribute("src", filename)
}else if (filetype == "css") { //if filename is an external CSS file
var fileref = document.createElement("link")
fileref.setAttribute("rel", "stylesheet")
fileref.setAttribute("type", "text/css")
fileref.setAttribute("href", filename)
}
if (typeof fileref != "undefined")
document.getElementsByTagName("head")[0].appendChild(fileref)
}
loadjscssfile("[URL]", "css");
loadjscssfile("[URL]", "js");
loadjscssfile("[URL]", "js");
//write GUI to DOM
document.write("<input id="calendar" type="text" />");
document.write("<input id="btnCheck" type="button" value="Check" />");
document.write("<br /><div id="result" />");
//Jquery to add calendar to textbox added above
$(function () {
$("#calendar").datepicker({
showOn: "button",
....
....
});

So if the jQuery/UI/CSS hasn't loaded, the script will get here, without having added the jQuery calendar to the textbox.