VB Overload resolution failed

I having this error during compile and i have no idea why~ Anyone can solve the problem pls.

Overload resolution failed because no accessible 'Add' can be called with these arguments:
    'Public Sub Add(entity As System.ServiceModel.DomainServices.Client.Entity)': Value of type 'LotusUniversityAd.Lecture' cannot be converted to 'System.ServiceModel.DomainServices.Client.Entity'.
    'Public Sub Add(entity As Web.Lecture)': Value of type 'LotusUniversityAd.Lecture' cannot be converted to 'LotusUniversityAd.Web.Lecture'.  C:\Users\Carson\documents\visual studio 2010\Projects\LotusUniversityAd\LotusUniversityAd\Views\Home.xaml.vb

This is the code behind

Private Sub addMem_Closed(ByVal sender As Object, ByVal e As System.EventArgs)
            Dim mem As RegistrationPage = sender
            If Not mem.NewMem Is Nothing Then
                Dim _OrganizationContext As OrganizationContext = LectureDataSource.DomainContext
                _OrganizationContext.Lectures.Add(mem.NewMem)

                LectureDataSource.SubmitChanges()

End If

This is the XAML code

<navigation:Page 
    xmlns:dataForm="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Data.DataForm.Toolkit"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:navigation="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation"
    xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" x:Class="LotusUniversityAd.Home"
    mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480"
    Title="Home"
    xmlns:ds="clr-namespace:LotusUniversityAd.Web"
    Style="{StaticResource PageStyle}" xmlns:riaControls="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.DomainServices">

<riaControls:DomainDataSource Name="LectureDataSource" QueryName="GetLecture" AutoLoad="True">
            <riaControls:DomainDataSource.DomainContext>
                <ds:OrganizationContext/>
            </riaControls:DomainDataSource.DomainContext>
        </riaControls:DomainDataSource>
    </Grid>
</navigation:Page>

mem.NewMem is of type LotusUniversityAd.Lecture, you likely have a DomainContext Insert method which takes a type of LotusUniversityAd.Web.Lecture (if not you will need one).

Note the difference in namespaces. If you resolve this, your error will likely be rectified. If you have defined the lecture class in your silverlight project, ensure it is within the Web namespace and defined as a partial class.