As of June 24, 2016 Canon EOS 1D-X Mark II is supported by Digital Camera RAW Compatibility Update 6.20 available in App Store Updates section.
If you previously imported images from an unsupported camera into Aperture then after you install the Camera RAW Compatibility Update, the unsupported image thumbnail is still displayed.
That’s because Aperture will not automatically update images to use the latest Camera RAW version. Doing so could change the look of an image you had previously edited. Obviously, Apple should have thought of the case where images were previously unsupported by Camera RAW.
To reprocess the images using the new Camera RAW in Aperture, follow these steps.
1. Select one or more projects that contain the images. There is no need to select the individual images; the following command operates on all images in the selected projects.
2. Under the Photos menu, select “Reprocess Originals…”
There are two choices to make; I chose “All photos” and “Reprocess existing images”.
3. After processing is complete, you will be able to view and edit the images in Aperture.