If you want to use jquery in magento plug-in development you can not use it directly , because magento is based on prototype java-script library but thanks to jquery this awesome library has a mode “no conflict mode” . what you have to do is
First define the noconflict() function
jQuery.noConflict();
and after that change the $ of the jquery to the jQuery as shown below :
jQuery(document).ready(function(){
jQuery(“#paysel”).change(function() {
// write code here for jQuery.
});
});