The way to do this with the latest version of the C# driver is:
var adminDatabase = server.GetDatabase("admin");
and then either:
var resut = adminDatabase.RunCommand("commandName");
or
var command = new CommandDocument { ... };
var result = adminDatabase.RunCommand(command);
Sorry for the inconvenience, but as mentioned in the release notes this change was made because continuing to support the admin specific helper methods when authentication was involved would have required a large number of new overloads.