If you go through the link you have provided, the solution is there itself.
It says like below...
After this step user is redirected back to your website (http://www.yourdomain.com/oauth2callback). Following is this callback code. Its purpose is to get a refresh-token and an access-token:
WebServerClient consumer= new WebServerClient(server, clientID, clientSecret);
IAuthorizationState grantedAccess = consumer.ProcessUserAuthorization(null);
string accessToken = grantedAccess.AccessToken;
An access token is usually valid for a maximum of one hour, and allows you to access the user’s data. You also received a refresh token. A refresh token can be used to request a new access token once the previous expired.
Sample Project links -
Refer dotnetopenid / samples / OAuthConsumer / GoogleAddressBook.aspx.cs
] for sample page and dotnetopenid / samples / OAuthConsumer
] for full project.