My best bet would be it either clipping on, or replacing the battery pack (like the above poster said). As for actually connecting to the system, there is an IR spot on the left side of the 3DS. Or maybe it can connect wireless like a bluetooth device.
Currently Let's Playing: Super Mario Galaxy 2 http://www.youtube.com/user/DTFaux
I saw a mock picture of what it is rumored to look like and if i were to guess, the only thing that makes sense to me would be the 2nd slide pad would work something like the 3ds docking station. Those two small gold plates that the docking station pins charge the 3ds might be also used to supply power to the slide pad attachment through some firmware update maybe?