jGrowl (and jQuery) don't run on the servers, they run on the clients. Because of that, I'm thinking your problem is elsewhere. You could back up, and try the following code to be sure jQuery and jGrowl are functioning properly.
$(function(){
$.jGrowl("I work!");
});
Also, if you could post a simple example of how you're using jQuery + jGrowl, that would help diagnose the issue as well.
What browsers have you tested this in? Are you absolutely positive you are referencing the plugin-source in your page? Try copying/pasting the reference source from your code directly into your url.