It attaches by using a foam padded sleeve that slides over the LCD screen of either the C300 or C500. The sleeve is secured into place with a bar clamp. It also includes a unique monitor lock clip to keep the LCD screen from moving forward when the Z-Finder is pressed against your eye.
The Z-Finder for the Canon C300 and C500 retails for $485 and is expected to ship mid-March. Check it out here at B&H Photo.
You can see the new Z-Finder and other Zacuto C300 and C500 accessories in the video below.