You have two possibilities to install.
Download from this page the zip, extract and place the folder Pacifica on home/user/.icons or usr/share/icons
Add the official PPA from launchpad for elementary OS, Ubuntu (Precise, Quantal, Raring, Saucy)
sudo add-apt-repository ppa:fsvh/pacifica-icon-theme
sudo apt-get update
sudo apt-get install pacifica-icon-theme