In most example code online, an object is loaded from the database, modified and stored back to the database.
If you are in a new dataContext and you have an in memory object you wish to store in the database you can use the attatch method of the table that the object belongs to.
DataContext dc = new DataContext();
dc.MyTable.Attatch(MyObject);
dc.SubmitChanges();
Obviously the MyObject needs to be of a type, or of a derived type from the type expected by the table.