In your web page:
<!-- include jQuery --><!-- include BlockUI -->
Everything you need to know can be found here: http://jquery.malsup.com/block/
Copyright © 2007-2013 M. Alsup.
You may use either license. The MIT license is recommended for most projects because it is simple and easy to understand and it places almost no restrictions on what you can do with the plugin.
If the GPL suits your project better you are also free to use the plugin under that license.
You do not have to do anything special to choose one license or the other and you don't have to notify anyone which license you are using. You are free to use the BlockUI plugin in commercial projects as long as the copyright header is left intact.