Are you sure it's rar files and not a binary split or something else like that?
Rar files uploaded here are no different than from anywhere else so either you're using a really old version of your software that doesn't support newer rar features or you're not dealing with rar files.
If it ends with .001, .002, etc., then it's usually a binary split and you can either use a command to join them or some software. You can check the pinned thread "How to join video files." in this section for suggestions of software to use, not too sure what's available on mac.
If you can't figure it out, tell us exactly what the filenames of what you're trying to open/merge are so we don't have to guess what the issue is.