If you mean that you did install it with "brew install ns-3", then there are some additional quirks.
While the homebrew package should work, I have no idea whatsoever about *where* it installs it.
I know there's something in /opt/homebrew/Cellar/ns-3, but beat me if I know where the sources are (if they're there at all).
Best suggestion: use the links posted by Gabriel and follow the instructions. The required packages can be installed using homebrew, of course.
As a side note, homebrew installs gcc as a ns-3 dependency, which is not at all required (if you have Xcode installed).