I added a field a field "Accept Terms" like it is descriped here:
http://deadlytechnology.com/silverstripe/membership-module/

But there is no error shown when they checkbox is not selected. I choosed requiered for the field in BE. Email is send with Activation Key. Now when the User tries to activate, comes up an error "Page not found".

Does anybody know how to validate if the checkbox "Accept Terms" is checked directly in the form?

Here my Seller Class:

<?php
class Seller extends DataObjectDecorator{
	function extraStatics(){
		return array(
			'db' => array(
				'AcceptTerms' => 'Boolean'
			)
		);
	}
}
?>

And the line in config:

Object::add_extension('Member', 'Seller');