By default, there are no rules set for a new promotion, unless you have selected a pre-built promotion. To show a tab, you'll have to go in and add it as a rule.
1. Once you enter the promotion you'd like to edit, click on "Promotion Rules"
2. Here you will find rule options you can add to your promotion. To show your tab, click on the blue "Add New Rule" button.
3. Name your tab rule and select "Show Tab" under the Action dropdown menu
3. Select your rules options to determine when to show the tab. To learn more about basic rules settings check out this article. Hit "Save" and then "Publish" to finalize the promotion!