Im trying to use this plugin:

http://www.marghoobsuleman.com/jquery-image-dropdown

I cannot get it to work in my file. I copied and pasted the code from the example files and made the necessary links to the js and css files just to test, and i still cant get it to work. Everytime i run it, i get this error:

Error: Result of expression '$(".mydds").msDropDown' [undefined] is not a function

The script im using to run the file is:

<script language="javascript" type="text/javascript">

function showvalue(arg) {
    alert(arg);
    //arg.visible(false);
}


$(document).ready(function() {

try {
        oHandler = $(".mydds").msDropDown().data("dd");
        oHandler.visible(true);
        //alert($.msDropDown.version);
        //$.msDropDown.create("body select");
        $("#ver").html($.msDropDown.version);
        } catch(e) {
            alert("Error: "+e.message);
        }
})

</script>

Why am I getting these errors and how can I fix it?