After your selected the External SD card, did you confirm it is the same size as the Micro SD card you have in the tablet. Many Android tablets also have a external stroage that can be deceiving, but the size will be different so that is a tell tell sign.
You might also try to select the External SD card and then touch the Download or Update button at the lower screen. If this does not work, then I'll have to bring Brian into this as I'm stumped as to why you are not seeing the files on the External SD card.