To compile Colobot on MacOS X, you need to first get Developer Command Line Tools for OS X, which you can get for example from Apple Developer website.
After installing Developer Command Line Tools, you should have basic tools like clang and git installed. After that, you can grab other required packages with Homebrew. So as in instructions on the project page:
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
And then:
brew install cmake sdl2 sdl2_image sdl2_ttf boost glew physfs flac libsndfile libvorbis vorbis-tools gettext libicns librsvg wget xmlstarlet
Gettext is installed in separate directory without adding the files to system path, so in order to get it working normally, you should call also:
brew link gettext --force
If you've installed everything correctly, the simple way of compiling Colobot with CMake should work:
git clone --recursive https://github.com/colobot/colobot.git
mkdir colobot/build
cd colobot/build
cmake ../
make
You can then build a Colobot drag-n-drop package
sudo make package
And open the package:
open colobot-*.dmg
Once opened, drag the Colobot application and drop it in the Application directory. Then just launch it as any other application.