It looks like you expect the guid value to be automatically generated for you right at the time of input. It looks like this column has DEFAULTof NewID().
LINQ To SQL table, . , , . , INSERT .

:
Customer c = new Customer{FirstName="foo"};
db.Customers.InsertOnSubmit(c);
db.SubmitChanges();
string someGuid = c.MyGuidField.ToString();