I had a similar issue (also on Mac here). Running...
cabal v1-update
cabal v1-install tidal
...seemed to properly install 1.51 for me. The plain 'cabal update' etc led to all sorts of issues. @yaxu talks about this at the beginning of the latest live stream, with a few different ways of upgrading.
Only issue was then Atom complained about 'multiple packages', but that can be fixed easily as detailed here:
https://club.tidalcycles.org/t/live-stream-6-tidal-1-5-0-release-party-and-q-a/1152/6?u=dand