Just FYI..Not sure which windows you are running ( I am assuming Windows 7 by the looks) .
Based on the driver and hardware, windows will allow you to select 24bit if it thinks THE DEVICE IS Capable. Example:
So you want to look at the AUDIO DEVICE it references and make sure you have the latest drivers
AND check your USB...If this is a USB device, it needs to ShOW as a USB device (USB AUDIO or the name of the DAC) - NOte how I just added a headset
So if I go to sound the highest it will let me select is 16BIT - That's all the DEVICE is capable of
So unless they give you a special driver that says I AM XYZ DAC CAPABLE OF 24 BIT, the device will not do it otherwise