After adding a new Mailchimp widget from your dashboard, click Connect via Mailchimp on the connection panel.
You'll now be asked to log into your Mailchimp account. Inside the pop-up window, enter your MailChimp Username and Password.
Click Log in to authorize your account.
To connect a different Mailchimp account to the one you've already connected to Geckoboard, see connect multiple accounts to data sources.