Twenty is a 2.1 audio device. It outputs two stereo channels and a mono sub-woofer channel.
If you have an AirPort express, it will look like one of the two units in the below photograph:
The unit on the left is the Original Airport Express (1st Gen), which docks with Griffin Twenty Audio Amplifier. The unit on the right is the New Airport Express (2nd Gen) which is compatible with Griffin Twenty Audio Amplifier but does not dock.
Twenty uses a standard IEC C7 (or IEC-C7) AC power cable. You can typically find these online through dealers such as Amazon.