Where is this "form designer"

You need to enable the Form Designer plugin separately; it will then be a submenu for the Constant Contact menu.

For that functionality, check out this post:
https://www.ads-software.com/support/topic/create-an-unstyled-sign-up-form?replies=5#post-3919523

I have the same problem as jamesdonegan.
And i have Form Designer plugin installed too. But in the form designing section i cant find any way to add checkbox to enable “”Join my mailing list” function. Please describe appropriately how to enable that. In the form what i find is all the textbox related fields. no field for checkbox.

Sorry to throw you guys under the bus, but if you change to the Contact Form 7 plugin, it works like a charm.