.corner error, just doesnt work!!

.corner error, just doesnt work!!

hi, I am not pretty expert in jquery, i am a begginner, still I cannot understand simple things, i have read a lot but still i keep getting problems, i am using a jquery's plugin for round corners  http://jquery.malsup.com/corner/ and i am using them like this

<script type="text/javascript" src="js/jquery-1.4.2.js"></script>
<script type="text/javascript" src="js/jquery.corner.js"></script>
<script>
$("#redondeada").corner("30px");
</script>

where #redondeada is a id div from a css that I am using, but still when i run this code i get with my firebug a problem
"








$("#redondeada").corner("30px");" is not a function.... i proved using jQuery instead the $ but still no results, i am working on joomla (which uses mootols) and sometimes i get conflicts, but most of all time it just doesnt work, its awful, someone got same problem? please help!