Your image is made up of intensities, they are looked up using the volumeScalarOpacity function in this example. So if it finds black (0 intensity) while sampling your image it will look up its corresponding opacity (0.0 is transparent, 1.0 is fully opaque). So assign opacity appropriately.
Example: If air/black is 0.0 and your structure lies at 100.0. Then you could set (0.0, 0.0) and (100.0, 1.0), where (intensity, opacity). Note that the function is linear and if it samples 50.0 you will get 0.5 (half transparency)