If you haven't used MailChimp before you'll want to sign up and create an account with them first here: https://mailchimp.com/signup/
When you have created your account have a run through their getting started guide: http://mailchimp.com/resources/guides/html/getting-started-with-mailchimp/ which is pretty nicely laid out. You'll want to import you customer emails from Synaps using the following procedure.
- In the Reports page in Synaps, select the report "Customer Emails List" under the Customers category
- Once the report has been generated click the "Export as Excel" button at the top (this will download a copy of the report to your Downloads folder or the folder you have set to receive downloads)
- You can open the Excel sheet and then either Save it as a CSV (comma delimited) File and upload it to MailChimp through it's uploader (Use the "Upload a File" option), or Copy and Paste your list straight from Excel into MailChimp (Use the "Copy/Paste your List" option)