Click on the Location button. You will be taken to this window:

If 503 is not entered into the Area Code box, type it in now. After that, click on the Area Code Rules button, which will take you to this window:

Check the box labeled Always dial the area code (10 digit dialing), and click on OK. You will be taken back to the Dialing Properties screen:

Note the Number to be dialed now is a 10 digit number. Click OK and you are ready to connect using 10 digit dialing.