Express Talk is not like Skype where the software is the service.
Express Talk is like buying a phone from your local shop, it doesn't provide you with any service on its own, you need to connect it to the telephone line from your service provider.
It is designed to interface with most SIP based VoIP providers to allow you to make and take phone calls through the Internet.
So your first step is to get yourself a VoIP provider, then you can set-up Express Talk to make calls using that VoIP provider.