Adding CampaignActivityItem in CRM 2011 gives 403 error

Question

I am creating a Silverlight 4 web resource in CRM 2011 using the Otganizational Data service. I am able to create Campaigns, Marketing Lists, and Print Activities as well as linking the lists to the campaigns. However when I go to link the marketing lists to the print activity using the CampaignActivityItem I get a 403 Forbidden error. Below is the code I am using.

        Models.CampaignActivityItem activityItem = new CampaignActivityItem()
        {
            CampaignActivityItemId = Guid.NewGuid(),
            ItemId = EmailMarketingList.ListId, //Id of my marketing list I have 
                                                //already created
            CampaignActivityId = new Models.EntityReference()
            {
                Id = MyCampaignPrintActivity.ActivityId, //Id of the print activity
                                                         //I have already created
                LogicalName = "CampaignActivity",
                Name = "CampaignActivity"
            }
        };

        context.AddObject("CampaignActivityItemSet", activityItem);

        context.BeginSaveChanges(System.Data.Services.Client.SaveChangesOptions.ContinueOnError, OnChangesSaved, context);

    private void OnChangesSaved(IAsyncResult result)
    {
        // Use the Dispatcher to ensure that the 
        // asynchronous call returns in the correct thread.
        OnUiThread(() =>
        {
            try
            {
               DataServiceResponse response = context.EndSaveChanges(result);
            }
            catch (DataServiceRequestException ex) // Errors with code=403       
                                                   // message=Forbidden
            {
                WriteOperationResponse(ex.Response, "ListLink");
            }
            catch (InvalidOperationException ex)
            {
                MessageBox.Show(ex.Message);
            }
            finally
            {
            }
        }
        );
    }

Any direction on what I am doing wrong would be greatly appreciated. Using the context I am able to perform actions on other objects but not the CampaignActivityItemSet.