Hi,
There is no configuration to do in the checkout.
The "authorize_end" view starts with this code
<?php if ($this->payment_params->api == 'dpm' && @$this->payment_params->iframe){
$url=urlencode(base64_encode(json_encode($this->vars)));
?>
<iframe name="frame" scrolling="auto" height="1000" width="660" Frameborder="no" src="<?php echo $this->vars["x_relay_url"].'&iframe='.$url;?>"></iframe>
<?php return;
} ?>
And in the onAfterOrderConfirm function of the plugin, the variable "iframe" is set to "1" if the API is not "AIM".
So unless you modified or create an override for the view, the iframe should be displayed.
Regards,