Launch Installer by pressings its icon on your SpringBoard.
Press to select the Install tab at the bottom of the screen
Press to select System from the list of Categories.
Press to select the BSD Subsystem package from the list of System packages.
Press the Install button at the top right of the screen.
Press the large Install button that appears.
A Notice will appear telling you not to use passwd in 1.1.3. Click the large OK button.
Once the installation is complete you will be returned to the Categories List.
After finishing the BSD Subsystem install you will most likely want to install OpenSSH from System category if it hasn't been installed already.