If you're logged into the Google Voice page, click the "CALL" button, enter the number and Google will dial your phone, then connect you to that number; the "TEXT" button sends text messages directly. You can also place a call from within Gmail by clicking the phone-icon button in the leftmost column.
Why can't I call from my Google Voice number?
Make sure you have a Google Voice account and phone number. Google Voice is only available in the U.S. If you have Google Voice,make sure that the email address you're using to sign in to Hangouts is the same as your Google Voice account. Make sure you've turned on ringing for incoming calls.