The media controls are only available when using the USB connection on your keyboard, so both connections need to be plugged in to your computer. If you are using a Das Keyboard with a Mac you have to hold the Fn key while pressing the corresponding media key (in blue) in order for those functions to work. Many web-based programs don't support all media controls. If they don't work in a computer-based software such as iTunes please check for any updates to your computer and try the keyboard on a different computer to see if it has the same behavior.
The brightness controls will only work with an Apple monitor because most PC monitors don't support brightness control from a keyboard. A Bluetooth keyboard connected to your Apple computer may also interfere with the brightness controls.