form validation is done using protype by default. If you want to use jQuery form validation you need to create your own form (tutorial available: [url]http://www.ssbits.com/using-jquery-for-form-validation/ [/url]) or use the module UserForm 0.2.x to create a form.
Did you check your sites sourcecode in your browser after page is rendered? Though it's more comfy to use firebug's "console" tab.
Did you check for version missmatch of your and the implicit loaded jquery.js?