You'll have to connect the device to your PC and then enable MTP mode. You can do this from the Settings menu on your iRiver device.
It's also recommended to be connected to the Internet, because in the event that Windows will need additional drivers, they will be installed from the Windows Update section automatically.
The download section of the iRiver website doesn't contain the necessary drivers, but I had a friend who purchased an iRiver device and was able to connect it to PC without drivers. Just make sure to enable the MTP connection in the Setting menu.