Follow the instructions below in order to locate your Mailchimp list ID which you can then drop in to the Sign Up form on your landing pages.
Log into your mailchimp account, then click on the Audience tab.
Check that you have selected the audience you would like the contacts to be added to within the Current Audience dropdown
Click Manage Audience and select Signup Forms from the dropdown
Click on Embedded forms
You do not need to change any settings on this page, scroll down to the section with the header: Copy/paste onto your site:
Within this code you should copy the text highlighted below within the speech marks following form action
e.g. <form action="https://yourcompany.us11.list-manage.com/subscribe/post?u=f5d8e779a9ef4bf56908a5178&id=cdee62a635"
You should then paste this code into the Mailchimp Form Action field within the landing page.