Q:

Cannot find all the types required by the async modifier

I've checked the several questions I found online on this matter and still I get this error.
I'm using Silverlight 5.0 and .Net Framework version 4.5 with crm 2011 and this is my function below:
async private void getDatafromCRM()
{     
    // ..... code to prepare the query expression
    IOrganizationService service = SilverlightUtility.GetSoapService();
    var response = await service.RetrieveMultiple(query);
}

The error I get :

(extension)
  System.Threading.Tasks.TaskIOrganizationService.RetrieveMultiple(QueryBase
  query)
Error:    Cannot find all types required by the 'async' modifier. Are
  you targetting the wrong framework version, or missing a reference to
  an assembly ?

I installed the "Async Target package and i got the message that it was added to my solution, why am i still getting this error ? 
I'm using async in my silverlight project not in the web application project. I noticed that the package was added to the Web project. 

A:

You must add Microsoft.Bcl.Async to your Silverlight project. Do not add it to your web project.

