If you have not already downloaded the POSTed Development Kit, go ahead and do so. You will find it by going to your Steam Library, clicking on "All Games" and picking the "Tools" option.
Once the tool is downloaded, right-click on it, select "Properties", then "Local Files", then "Browse Local Files" to find the install location. Once you have done this, create a shortcut to the install location on your desktop, or in a place you can easily access for future reference.
Inside the install location you will find a zip file containing the UnrealScript source code. Extract the entire archive on top of your install location. This will create new folders called AIPack, AWEffects, AWGame, and so on.
Next, go into the System directory, and inside System create a shortcut to the command prompt, cmd.exe. Once you have done so, right-click on your new shortcut, hit "Properties", and delete the value for "Start in:". This will give you a handy command prompt shortcut that will start you directly in your System folder, which you can then use for launching ucc.exe for compiling scripts, as well as Postal2.exe for quick and easy testing.