Go to the "System Preferences".
Click on "+" button and select "VPN".
Make changes like this image and click on "Create" button.
On this screen, you have to specify either hostname or IP address of your VPN accont. Add your username in "Account Name" and click on "Authentication Settings...".
Note: To see your server IP address, login here with your username and password.
In authentication windows add your VPN password in the "Password" and then click "OK".
After return to the previous screen, click on "Advanced..." button and check the "Send all traffic over VPN connection" and click the "OK".
Now you are ready to connect. To connect VPN just click on "Connect" button. Or ou can also initiate a VPN connection by clicking the VPN icon on the menu bar.