To link your email marketing with myFlashStore, you'll need a Mailchimp or Aweber account.
First head to Dashboard -> Settings -> Email Marketing.
For Aweber, click the green button to link your account, and give permission for access.
For Mailchimp you will need an API key. To generate this key, follow this guide.
Once you've done either (or both) of these, your customers can now be automatically added to your mailing list after a successful purchase.