The issue of Famitsu that introduces Nintendo's new add-on 3ds thumb pad is on sale in Japan. Don't live in Japan? Like Microsoft Paint? There is this.
Kotaku Japan picked up a copy of the magazine, and using the images in Famitsu, recreated the 3DS Expansion Slide Pad. In the above image, Kotaku Japan wrote "slot". Charming handwriting!
In the below image, next to the large circle it says triggers. In the middle is the "cover" or "フタ" that blocks the cartridge slot.
The drawing isn't perfect -- cute, but not perfect. Nintendo will hopefully release official art. Until then, this homemade image from Kotaku Japan might make it easier to visualise the 3DS Expansion Slide Pad.
イラスト解説：ニンテンドー3DS拡張パッドはこうなってます [Kotaku Japan]