Select the list where the message is located by clicking the Manage button from your lists on the dashboard.
Click the Messaging icon in the list navigation bar in the top right hand corner of your list homescreen.
If you're looking to find the Message ID for an after opt-in message that is being sent to new subscribers, you'll want to click on the link that says Scheduled, below the green New Message button. Find the after opt-in message you're looking for and click the Report link for that message. If you're looking to find the Message ID for a message that was sent to all of your subscribers at the same time, you'll want to click on the link that says Sent, below the green New Message button. Find the sent message you're looking for and click the View Report button for that message.
When on the message page, you'll see that the web address should be something like https://app.tatango.com/broadcasts/70095-BOBS/recurring_messages/4645, and for this specific message, their Message ID would be 4645. Now it's time to go find your own Message ID, good luck!