In the demo we use a plain text widget and add this:
<p style="margin-bottom: 10px;">Sign up for our newsletter:</p>
<div id="mc_embed_signup">
<form action="http://edcousins.us2.list-manage1.com/subscribe/post?u=a3950048c69f7be72a27013a9&id=68a002176a" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate>
<input type="email" value="" name="EMAIL" class="email" id="mce-EMAIL" placeholder="Enter your email address here" style="margin-bottom: 0;" required>
<div class="clear"><input type="submit" value="Subscribe" name="subscribe" id="mc-embedded-subscribe" class="button" style="visibility:hidden;"></div>
</form>
</div>
You need to get your form code from mailchimp
Let us know,
Thanks,
laranz.