Here you can find the official C# library source code for Webmaster Tools Data API:
http://google-gdata.googlecode.com/svn/trunk/clients/cs/src/webmastertools/
And here you can find examples of usage in the form of unit tests:
http://google-gdata.googlecode.com/svn/trunk/clients/cs/src/unittests/webmastertools/
More specifically, examples of authentication, QuerySites, QuerySitemaps, QueryKeywords, etc.:
http://google-gdata.googlecode.com/svn/trunk/clients/cs/src/unittests/webmastertools/WebmasterToolsServiceTest.cs
Note also there are some bug reports, your mileage may vary:
http://productforums.google.com/forum/#!topic/webmasters/gh7vCzYfm6A
If you're still getting 403 Forbidden
, the reason behind this might be the 2-Step Verification installed on your Google account. If that's the case, try generating a dedicated password in the Application-specific passwords section of your Google account, and use it with GDataCredentials(this.userName, this.passWord)
.
Alternatively, you may try adding sub-accounts at https://www.google.com/webmasters/tools/user-admin and use of one of them. The 2-Step Verification concern may apply to those accounts, too.