Follow the link in the "About this Email Service" for instructions on obtaining an API Key
Paste your API Key into the first box
After a second or two, your MailChimp lists should show up in the Default List box (if not, check your API Key; sometimes even a space before or after they Key can cause problems with MailChimp)
Select your Default List
Check the Active box (DO NOT FORGET THIS STEP)
That's it! You're now setup to use MailChimp as a destination for Landing Pages.