Why does a newly created EF-entity throw an ID is null exception when trying to save?

I´m trying out entity framework included in VS2010 but ´ve hit a problem with my database/model generated from the graphical interface.

When I do:

user = dataset.UserSet.CreateObject();
user.Id = Guid.NewGuid();
dataset.UserSet.AddObject(user);
dataset.SaveChanges();
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{"Cannot insert the value NULL into column 'Id', table 'BarSoc2.dbo.UserSet'; column does not allow nulls. INSERT fails.\r\nThe statement has been terminated."}

The table i´m inserting into looks like so:

-- Creating table 'UserSet'
CREATE TABLE [dbo].[UserSet] (
    [Id] uniqueidentifier  NOT NULL,
    [Name] nvarchar(max)  NOT NULL,
    [Username] nvarchar(max)  NOT NULL,
    [Password] nvarchar(max)  NOT NULL
);
GO

-- Creating primary key on [Id] in table 'UserSet'
ALTER TABLE [dbo].[UserSet]
ADD CONSTRAINT [PK_UserSet]
    PRIMARY KEY CLUSTERED ([Id] ASC);
GO

Am I creating the object in the wrong way or doing something else basic wrong?

On the model for the fields which is defined as key, add the following attribute. This key which is not marked as IDENTITY in database, Entity Framework give the exception

[Key]
[Display(Name = "Id")]     
[DatabaseGenerated(System.ComponentModel.DataAnnotations.DatabaseGeneratedOption.None)]
public int Id { get; set; }

You shouldn't have to set the ID property manually. It should be set automatically when you save. I assume you are using the standard template for Entity Framework, and not the Self-tracking entities template or POCOs template. If so, something along the following lines would seem more appropriate:

User user = new User();
dataset.AddToUsers(user);
dataset.SaveChanges();