In Mac OS Sierra 10.12 you are not able to see your scanner sources in the source list
Apple did not include the ICA drivers in this version of the OS.
Upgrade to a newer version of the OS as 32-bit ICA driver support is back in as of the 10.12.1 beta version.
As this only affects the ICA drivers which is what we use by default on Mac you may change the image capture driver type to 0 to specify TWAIN only devices. However TWAIN on Mac is not as widely adopted.