The first time you run your softphone there are no sip accounts running on it already. The softphone will automatically ask for creating sip accounts by opening a new sip account dialogue.
Click on "Add". A new dialogue will be opened.
Enter the following in the field mentions by the red rectabgle in the above screenshot
Display name: Enter your name
User name: Enter your username
Password: Enter your password
Authorization user name: Enter your username
Thats it. Press the ok button. You will see a new account has been created in the "Sip Account" dialogue. When you close this dialogue you will see on the screen of the softphone that it starts registering. It will show on the screen when its registered.
You can make calls now using the number pad on the softpone and then pressing the green phone button. Make sure you have attached a mic and a headphone or speakers with your pc.