Mailpoet checkbox at comment form

Hello,
I’m using Mailpoet (https://wordpress.org/plugins/wysija-newsletters/) for my newsletters.
In the Mailpoet-settings there is the option to have a checkbox at the comments form so visitors can subscribe easily.
This checkbox isn’t appearing though when activated in the settings.
Mailpoet support tells me this is because of the way my theme (Nirvana) is coded, so I should get in touch with the makers.

Is it possible to have the checkbox at the comment form?

Cheers, Capo