Please try to install this file management app for help: click here.
How to install the app: You can copy the apk file of the qualified app to the player, then open the File Management app, locate and click on the apk file to install it.
You would need to remove the white list limitation first: go to "Settings -> General > About device" and tap 7 times on 'Kernel version' to remove the limitation,and then reboot the player.
You could also install the ES file explorer app instead of using the file management app: http://fiio-file.fiio.net/whitelists/estrongs.apk