Q:

How do I install the GNOME sensors-applet package in Unity? 

I want to install sensors-applet onto my Ubuntu 12.04 computer.   The instructions on this (and other) webpages says to enter this command in a command terminal:
sudo apt-get install sensors-applet
This is a gnome applet and gnome isn't exactly obscure.   Why can sensors-applet package not be found? 
Before you say "no", note that I was able to execute "gnome-panel" to add a bar/panel along the bottom of my desktop, add 8 cpu-frequency applets to the bar (one per CPU core), and add gnome-panel to my startup applications.   So I've already done the basics, and believe all I need now is to be able to download and install the sensors-applet to have that temperature applet available on the panel too.   See here. 

A:

The sensors-applet package is not available in the 12.04 repositories.  As a matter of fact, it is not even in 11.10. 
The last release that had it was 11.04. 

A:

for 11.10 and earlier
The GNOME2 package for sensors-applet  is available for the following Ubuntu versions only:

Hardy Heron 8.04 LTS

Lucid Lynx 10.04 LTS

for 12.04
Sensors applet 3.0.  is not available from the 12.04 LTS repositories.  You may be able to install from e.g. the following ppa (untested by me):
ppa:darxus/sensors-applet

Alternatively you can always download the source from GNOME to compile and install from there. 
for 12.10 and later
The GNOME3 version of sensors-applet  is is at present only available from the Ubuntu repositories for Ubuntu >= 12.10.  Ubuntu 12.04 is not supported.