You could do this more simply as follows:

Graph g = new Graph();
URILoader.Load(gUriLoader.Load(g, new Uri("http://rdf.freebase.com/ns/en.roman_abramovich"));

URINode UriNode dob = g.CreateURINode("fb:people.person.date_of_birth");

ISelector<Triple> selector = new SubjectHasPropertySelector(g.CreateURINode(), dob)g.CreateUriNode("fb:people.person.date_of_birth");

foreach(Triple t in g.GetTriples(selector)g.GetTriplesWithSubjectPredicate(g.CreateUriNode(), dob))
{
    Console.WriteLine(t.ToString());
}

That gets you the date of birth as you require. Note the use of g.CreateURINode(g.CreateUriNode() to create a URI Node that refers to the Base URI of the Graph.

You could do this more simply as follows:

Graph g = new Graph();
URILoader.Load(g, new Uri("http://rdf.freebase.com/ns/en.roman_abramovich"));

URINode dob = g.CreateURINode("fb:people.person.date_of_birth");

ISelector<Triple> selector = new SubjectHasPropertySelector(g.CreateURINode(), dob); 

foreach(Triple t in g.GetTriples(selector))
{
    Console.WriteLine(t.ToString());
}

That gets you the date of birth as you require. Note the use of g.CreateURINode() to create a URI Node that refers to the Base URI of the Graph.

If you point out which bit of the documentation is unclear I can update it appropriately since I'm the developer of dotNetRDF so I'm keen to encourage adoption and improve documentation where needed.

With regards to Pierre's post dotNetRDF sends appropriate HTTP Accept headers so that if the URI does content negotiation to return RDF then it should get RDF back. I don't have enough reputation to put this as a comment yet.