When you reach the DESIGN CAMPAIGN step on the CAMPAIGN CHECKLIST, you will have already been guided through selecting the first message you want to play in your campaign. If you only want to play one message in your campaign, you are done with the DESIGN CAMPAIGN step and can move on to testing your campaign.
When you want to add additional elements to your campaign, simply click on the first message from the DESIGN CAMPAIGN screen and add the type of element that you want.