Yes, you would have to purchase the pack to access the FBX files, I don't know anything about Cinema4d, so I don't know how well this pack will work, as it's intended for Unity and Exported from Unity, and I don't know how the FBX files will be read in Cinema4d. I made the FBX files mostly for people that use Unreal Engine, and from what I heard from people that tried it, some got it to work, while others haven't.
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You'd have to have Unity installed to unpack a Unitypackage, with Unity installed have a Project open and double click on the package and it should import the files to Unity. I don't know if there's Unitypackage converter for Cinema4d, I've heard there's one for Godot, but I don't know much more than that.