Error: $("#test_addbtn").button is not a function – solution

If you are getting this problem, this is because button() is a part of jQuery UI 1.8. By default 1.7 is still used. Since 1.8 stable is being released we will switch over shortly, but for now here is the fix. Put this into your config.php to manually change version of jquery / jquery ui:

$config['js']['versions']['jquery']='1.4.2.min';
$config['js']['versions']['jqueryui']='1.8.custom.min';