The easiest way to install mappacks is by clicking the [Download Mappacks] button in the matchmaker. From there you can manage which maps you have installed and install new ones.
If you wish to manually install mappacks, follow these instructions. Before downloading any of the Maps or Mods, make sure that the most recent Matchmaker is installed first, otherwise the files below will not install properly! Download the files (right click on the link and save them) to your mappack directory which is accessible by going to the [Mods] window in the matchmaker and clicking on the [Directory] button.