Login to your dashboard. From the dashboard, select "Billing," then select "Add a Number." You'll be able to search area codes for numbers and select a number. Click "Add This Number," and you're done.
NOTE: If you've already reached your limit of numbers in your plan, it will cost you an extra $5 a month to add each number to your plan. You may want to consider upgrading to a higher plan if you think you may need additional numbers.