To write a disk image onto an SD card, you need to first download the image file onto your Mac or PC from here: http://www.raspbmc.com/download/
You can if you like use the UI installer, that is available for Windows and Mac, but as a Raspberry Pi user, it is useful to know how to create an SD card for the Pi for any disk image. For example you will need to do this when trying out a different distribution of Raspbian or Occidentalis.
So, scroll down the page until you get to the section Just want an image without a fancy installer?
Then click on the link for Standalone Image.
The alternative network image just contains enough of the system to then download the rest of itself once it is installed in the Raspberry Pi. The problem with this is that it requires a network connection to do this, which is fine if you are connected with a network cable, but does not work with a USB WiFi dongle.
By selecting the Standalone image, we will just get everything we need in one go.
Now you have downloaded the disk image, the process is exactly the same as for any other image file, so follow the instructions here on creating an SD card. http://learn.adafruit.com/adafruit-raspberry-pi-lesson-1-preparing-and-sd-card-for-your-raspberry-pi