error: while evaluating ‘callPackageWith’ at /Users/mitch/src/github.com/NixOS/nixpkgs/lib/customisation.nix:113:35, called from /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/top-level/all-packages.nix:17789:24:
while evaluating ‘makeOverridable’ at /Users/mitch/src/github.com/NixOS/nixpkgs/lib/customisation.nix:72:24, called from /Users/mitch/src/github.com/NixOS/nixpkgs/lib/customisation.nix:117:8:
while evaluating anonymous function at /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/games/2048-in-terminal/default.nix:1:1, called from /Users/mitch/src/github.com/NixOS/nixpkgs/lib/customisation.nix:74:12:
while evaluating ‘mkDerivation’ at /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:15:5, called from /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/games/2048-in-terminal/default.nix:3:1:
while evaluating ‘addPassthru’ at /Users/mitch/src/github.com/NixOS/nixpkgs/lib/customisation.nix:135:22, called from /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:166:7:
while evaluating anonymous function at /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:5:1, called from /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:167:22:
while evaluating ‘handleEvalIssue’ at /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:128:21, called from /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:203:17:
Package ‘2048-in-terminal-2015-01-15’ in /Users/mitch/src/github.com/NixOS/nixpkgs/pkgs/games/2048-in-terminal/default.nix:25 is not supported on ‘x86_64-darwin’, refusing to evaluate.

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowBroken = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowBroken = true; }
to ~/.config/nixpkgs/config.nix.

So I do b, and then:

error: Please be informed that this pseudo-package is not the only part of
Nixpkgs that fails to evaluate. You should not evaluate entire Nixpkgs
without some special measures to handle failing packages, like those taken
by Hydra.