This article explains how to enable SSH access to your pfSense box. Note that you normally will not need to enable this, it's typically only used for debugging and troubleshooting purposes.
In order to enable SSH, you'll need to go under the System -> Advanced menu in the WebGUI.
On pfSense 1.2.x, there was a box here to enter authorized SSH keys. On pfSense 2.x, these keys are added to the individual users under System > User Manager.
The SSH daemon may also be enabled or disabled from the console using Option 14 on the menu.