Also how it works, hint - it requires a battery
While there was absolutely no word about the 3DS Circle Pad addon at the Nintendo 3DS Conference 2011, it has appeared on the Nintendo Japan website with some new information about it, including a proper picture.

Posted ImageThe device, which will run on an AAA battery, has to be used while docked with the system; apparently it can’t run by itself. It’s set to be released on the 10th of December, the same day Monster Hunter Tri G comes out in Japan, and will be an optional accessory for 1500 yen. That’s about $20AUD.

The following games will support the add-on; funnily enough, no Nintendo games are listed.

Monster Hunter 3 (Tri) G
Biohazard Revelations
Metal Gear Solid Snake Eater 3D
Ace Combat 3D Cross Rumble
Dynasty Warriors VS
Kingdom Hearts 3D: Dream Drop Distance

As suspected, only games that support it will be able to use the second stick; if you don’t have the add-on, don’t worry, the games will still work. There will be a little logo on each game box that support it.