The sequence to make things work is:
1) Download the USB-DAC driver from ibasso.com
2) Install the driver (can be omitted, but I did install the driver first)
3) Unrar\unzip the contents of the driver archive in a separate folder
4) Turn the DX 90 on, go to Settings->USB settings-> set the "DAC" mode
5) Open your Control panel->Device manager
6) Connect DX 90 usb cable to your PC (USE USB 2.0, USB 3.0 IS NOT RECOMMENDED DUE TO DX90'S CONFORMANCE WITH USB SPECIFICATIONS ISSUES). After the DAP displays "DAC mode" on the screen you should see a new "Mango" device in the list of your audio devices in device manager.
BE WARNED! Mango device is displayed in the list just for a few seconds, then it disappears. So before the Mango device disappers from the list you need to select it and select "Update driver", select manual search for a driver directory and specify the folder you've previously unzipped the USB driver archive to.
After the driver is installed you should see the Mango device in the list of Playback devices and can enjoy DX 90 working in a DAC mode...with all the lags and sporadic reboots
)