The heat maps are designed to display at larger scales, so they are visible only when the map is zoomed out to the level 6 setting or greater.
To find level 6, use the (+) button on the map to zoom all the way till you can go no further (level 1). Then click the (-) button five times, to zoom out to level 6. You will be in the “aerial” view. The “overhead” view is available only at the level 1 or level 2 zoom settings.