Smuxi is in the nixpkgs/unstable channel, so you can simply install it on NixOS or macOS with this command:

$ nix-env -i smuxi
$ smuxi-frontend-gnome

Or run it from an ephemeral shell:

nix-shell -p smuxi --run "smuxi-frontend-gnome"

Or to run the text user-interface:

nix-shell -p smuxi --run "smuxi-frontend-stfl"

Or to run the smuxi-server:

nix-shell -p smuxi --run "smuxi-server"

You can also start Smuxi from the GNOME or KDE menu.

Smuxi's nix package is known to build and run on NixOS and macOS including x86 and Apple Silicon. If you run into issues, please open an issue at: https://github.com/meebey/smuxi