Here's how to get it up and running.
~$ sudo apt-get install lm-sensors hddtemp
The above installs the sensors software you'll need to access hardware sensors. Next, you'll need to configure lm-sensors.
~$ sudo sensors-detect
Just answer "yes" to every question and let lm-sensors handle everything for you. You'll need to restart afterwards so the service starts up. If you prefer, you could manually startup the service immediately.
~$ sudo service module-init-tools start
Finally, install Psensor.
~$ sudo apt-get install psensor
|The Psensor main window|
You can now run Psensor from the Unity Dash. A sensor indicator should also appear in the indicators area.
The above instructions were gleaned from this webupd8 post. There's instructions there for installing more recent versions of Psensor, or even the unstable release, if you're so inclined.