Advanced settings must be configured to allow for pop ups. 

1. Go to the Contest Engine Support tab
2. Click Advanced Settings
3. Check the Enable Popup Notifications? field
4. Click Save Settings

You must also have the Mini Leaderboard Splash field checked for the contest (For Classic SF users). To get to the Notifications tab, you must edit the contest.

1. Go to Contest Builder
2. Choose the Contest you need to edit
3. Click Edit
4. Go to Notifications and check the box for Mini Leaderboard Splash
5. Save