Tap the Dolphin icon on the toolbar in the bottom of the screen. If Auto Fullscreen is enabled and the toolbar is not showing, scroll up to display it.
Make sure that the toggle on the bottom of the screen is set to Sonar (not Gesture).
Alternatively, Dolphin Sonar can be accessed by shaking your phone. This option must be activated by accessing Sonar settings, see “How do I enable / disable shake to activate Dolphin Sonar”.