jQuery effect after pressing button
Drupal 6 version of JQuery is 1.26 and after install jquery_update it will be updrade to 1.3.2

If you want persist use jquery method likes effect maybe this article,but I think you can't using jquery 1.6 or upper in Drupal 6 .( I don't told you it's impossible but achieve this job is more struggle and maybe cause problem in other part of your site ).

maybe this question be helpful for you.