Hey there,
Thanks for posting. As you alluded to, I’m afraid that our WordPress plugin does not directly provide a popup feature. Alternatively, you may consider giving the MailChimp embed code a try, which does include an option to enable what’s called “evil popup mode”, which would provide the popup interaction you’re looking for. In some cases this may work on a WordPress site, but in other cases it may not. So if you do go this route, make sure to test thoroughly.
Here’s a link walking through how to use the embed code:
How can I add my signup form to my website?: http://eepurl.com/gOGN
If the above isn’t ideal, then the best bet would be to continue testing the MailChimp embed code on various plugins to see which ones support our forms and which do not. As far as plugins, I’m afraid that I don’t have any direct recommendations to which plugins would work best. A user not too long ago mentioned working with the Easy Modal plugin and using the MailChimp embed code with some success, so that may be one combination to test out. If you have any additional questions about our plugin, feel free to reply here and we’d be happy to help!