In 2D: Manipulate In-Game works by panning with the middle mouse button, and zooming with the mouse wheel. You do this directly on the game viewport, not in the editor. Set the mode in the Game tab to 2D for this to work (Input won’t work, the input will go straight to the game instead).
In 3D: Manipulate In-Game works by holding the right mouse button, then moving the mouse and using the WASD keys to move the camera. Like in 2D, you do this directly on the game viewport, not in the editor. Set the mode in the Game tab to 3D for this to work (Input won’t work, the input will go straight to the game instead).