Subscribe by email - success message not displaying

I have just installed the Subscribe by Email plugin and everything works nicely except the message saying you have successfully subscribed does not appear in any of the browsers I am using (Safari and Firefox on Mac in particular). Firefox complains that jQuery is undefined at line 653, I have also noted on the jQuery lines the closing </center> tag has a typo but I think that is just cosmetic. I am using version 1.0.4 of the plugin and I noticed there is another topic discussing this issue for IE8. I have tried the plugin in IE8 and get the same error.

Thanks for your help.