Attention: Your website must be a Joomla.org and NOT a Joomla.com website.
Login to your MailMunch account and copy the site code for your Joomla site.
Sign into your Joomla account and in your Control Panel click Templates located under the "Configuration" heading in the column on the left.
Click on your active site template name (indicated by a star next to the name). In this case, it is Protostar
Click on index.php from file list available in the column on the left side of the screen
Paste the MailMunch site code you copied earlier inside of the head tags within index.php. Once you have click Save & Close
The forms you have created in your MailMunch account are now live on your site