You can use ] dev <path_to_locally_downloaded_ShowCases>, before using ] add MLJ. If the local ShowCases.jl includes a .git folder, which is the case for e.g. git clone, but not if you download the code as a zip file, you can (and probably should) use ] add <path_to_local_ShowCases> instead.
(I was originally worried that you might need to locally clone MLJ.jl and adjust its Project.toml (and/or Manifest.toml), but that does not appear to be necessary. In fact, ShowCases does not even appear in the Project.tomlfile! 
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