The state of Meld on Macs is currently in flux. TLDR: currently, the best
way to run Meld is to install Homebrew and then run
brew install dehesselle-meld.
Update 2025-04-21: Starting today, it seems that Homebrew renamed the
dehesselle-meld cask to just meld, https://formulae.brew.sh/cask/meld.
So, brew install --cask meld should work (possibly after updating
Homebrew), and simple brew install meld might work too.