Dialogs work off a div with an id. So inside your <form> and around your <fieldset> you need to create a <div id = "dialog-form"> with the id you have assigned to the dialog open statement. You may or may not need the button() part of the current statement, depending on the rest your HTML. I presume the form section you posted is not all of your HTML.
Take a look at the documentation for a Dialog fired by a button, and use it as a model.
http://jqueryui.com/dialog/#animatedIf you don't want it animated, remove the show, hide options. The documentation has a paragraph as contents. You will replace that with your asp input statement.