The Analog Experience keyboards are powered via USB (whether from a computer, or from a universal USB power supply), therefore Arturia does not include an external power adapter (it says "optional, sold separately" on the box).
If you want to purchase an external power supply (from Radio Shack or similar store), you need to check the following specs:
- Output Voltage: between 9 and 12 VDC (DC, not AC)
- Current: 500mA (0.5A) or more
- Polarity: Positive Center pin (this is very important!)