You need to include the jQuery file in head section for executing the code.
How to include jQuery in ASP.Net project?[
^].
You can include the script file directly in your page/master page, etc using:
<script type="text/javascript" src="/scripts/jquery.min.js"></script>
Us use a Content Delivery network like Google or Microsoft:
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
or:
<script src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.2.js" type="text/javascript"></script>