To get the button code for your MailChimp email:
In your Eventable dashboard, go to the Events tab
Click the Share Event button for your specific event
If you haven't already, select HTML as your email type
Select MailChimp from the dropdown menu
Copy the embed code
To paste the embedded code in MailChimp:
Go to Edit Template for your specific event
In the right hand panel, select Code
Drag the code block into the left hand panel
Delete the existing placeholder text, and paste the embed code (Control+V if using Windows, Command+V if using Mac)
Save & Close
Preview your buttons in the email by clicking Preview and Test