I think I see what you mean. Forget about QuickTime. It is no longer needed. In fact, if you are on Windows, you should NOT have it installed. It is a security risk now. So I suggest you uninstall QuickTime and be done with it, unless you are willing to live with the security risks. There are NO extra codec files to download or install for the new video engine inside Cubase (except Avid DNxHD which requires a license purchase from Steinberg). So the codecs that are needed to play back the supported video files inside Cubase are already built into the Cubase video engine, WITHOUT worrying about QuickTime. All you need to do is load a video into Cubase that is already encoded in a supported codec. In the case of ProRes, the file must be in a MOV container file (which it probably already is).
File > Import > Video File
Select the file and you will be good to go IF the file is supported. If Cubase won’t load it up, then there might be a file issue or some other issue going on, or it is not encoded in a supported codec. You’ll have to troubleshoot the file itself and find out exactly what container and codec was used when the file was created. It must be encoded in a format that Cubase can play back. That link I mentioned above lists the supported containers and codecs.
If you are having more trouble, then you need to use some sort of tool to confirm what your source file is… and/or re-encode it into a different codec that is supported. There are some other threads in the forum that deal with that situation.
Hope this helps a little.