Answer:
Accessing Street View depends somewhat on whether you're using it from Google Maps or Google Earth.
Street View in Google Maps:
Drag the orange "pegman" icon from the navigation control (on upper-left corner) onto the map to enter Street View mode.
Street View in Google Earth 6:
Drag the orange "pegman" icon from the navigation control (on upper-right corner) onto the map to enter Street View mode. Also if you're looking at ground from low altitude then double-click on a building or ground to enter nearest Street View photo from that location.
Street View in Google Earth 5:
Check the Street View layer under Layers panel then double click on photo icons that appear on the map.
For tips to navigate in Street View see related links for video tutorials.