I was hoping that Aperture 2.1.3 supported tethered shooting with 5DMKII, but it looks like NO. Even after upgrading to 10.5.7.
The camera is recognized, but says "BUSY" when trying to capture.
Aperture's tethered support doesn't work with any of the new Canon cameras. Aperture's tethering support relies on a standard tethering protocol. Canon used to support this, but they have moved to a proprietary solution starting with the Mark III cameras (so the 1D and 1Ds Mark III, the 50D, and the 5D Mark II, and likely others). I'd think Aperture would completely need to re-implement tethering support to support the new Canon cameras, and I don't know if Canon even publishes their new proprietary spec.
So for now the solution is to use Canon's Capture app and set up a hot folder in Aperture, like was necessary in Aperture 1.x