Is it configured with your xbox? Also, why not use Horizons device explorer?

You can use that, but I would recommend using Horizon’s Device Explorer.
It’s just easier in my opinion.
Also, use this tutorial to configure your USB to your Xbox/PC;

That is probably the problem as it is most likely not configured.

Here’s a look at Horizon’s Device Explorer if you haven’t seen it:

From there, you can explorer all of your folders containing your contents:

Besides the previously stated above…

Have you tried running the program as Administrator? Right click XTAF and then select 'Run as Admin.'
I’ve had this issue before and it has fixed it.

For those asking why he does not use Horizons… I don’t use it often either. It is a lot easier to load up just a device explorer such as XTAF instead of waiting for Horizon to boot up. It probably differs from computers but it takes about 20 seconds to open Horizon compared to the 5 for another program.