This is quite simple to install and the process is almost identical on OS X and Linux systems. The first step is launching a shell prompt, this can be done on Mac OSX by opening Terminal, or either on the Linux command line or opening an X11 Terminal emulator.
Once you have Terminal up, you should proceed to download the installer by typing in the following commands:
wget http://svn.stmlabs.com/svn/raspbmc/release/installers/python/install.py chmod +x install.py
curl -O http://svn.stmlabs.com/svn/raspbmc/release/installers/python/install.py chmod +x install.py
Now we need to run the installation script with root privileges as we will be writing to an SD card. In some distributions you can do this by prefixing the command with sudo whereas in some you will need to su root. You should consult your OS documentation for more help on this matter. But in the case of a standard Debian install, one would run:
sudo python install.py
You will then be greeted with a screen similar to the one below. You are now using the installer. Simply follow the instructions and you should be fine.
Note, if you get an error similar to the one below, then you did not run the script with root privileges:
Raspbmc installer for Linux and Mac OS X http://raspbmc.com ---------------------------------------- Please re-run this script with root privileges, i.e. 'sudo ./install.py'