Hi Lucian,
I think you may be misunderstanding. Any field that you can create in MailChimp can be displayed in our form, exactly how it was in previous versions. With the new version you can no longer manage your merge variables from within the plugin, but now have to do so over on MailChimp.com.
Anytime you create or remove fields from a list on MailChimp.com, you should then see the changes take effect inside of the edit form page.
With all that being said, you should have no problems going in to your mailing list and creating any number of fields you’d like on MailChimp.com. Once you’ve done so you can then add them to the forms using the form builder. Then you can save the post and all of your form fields will then be displayed on the front end.
You cannot create additional fields that are not supported by MailChimp as MailChimp would have no idea how to handle the data.
What fields is it that you are having issues with? I have just run a few tests and can confirm that the Zip fields are working as intended.
Thanks,
Evan