Create MailChimp Campaign
From the top row of buttons, click Integrations > MailChimp.
Select your email signature and click "use this".
Select your MailChimp segment - making sure it is highlighted - and type your subject.
Next you will want to highlight the "message" box in the body of the email, and replace it with your email content.
Once you have created your email you will need to click "Save Campaign".
You will be redirected to the "Campaigns" tab; you can either send a test, send the email now, or schedule a time using the links on the right.