In order to print courier labels, you must have the printing application open and running in the background on your machine.
This should be left running at all times. However, we know that PC's need a reboot from time to time, so when you do so you will need to ensure that you have reopened the label printing application.
You should already have a shortcut to the App on your desktop. It will be called 'Courier Label Printer'. The current version of the application is CourierLabelPrinter18.104.22.168.
If you no longer have the shortcut on your desktop, you can just click on the 'Start' button on your PC and start typing the first part of the file name (CourierLabel):
Now that you have double-clicked the file to open it, you will be presented with the contents of the file. You need to select the Application and double-click to open it:
You will now have the application open and running. You can minimize the window and leave it running in the background: