Not sure if you have reviewed this - but I have had to add the provider factory to the machine.config file manually before being able to see the provider in Visual Studio.  It seems this is missed in some of the files.

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\machine.config

C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config\machine.config

    <system.data>
        <DbProviderFactories>
          <add name="InterSystems Data Provider" invariant="InterSystems.Data.CacheClient" description="InterSystem .Net Data Provider" type="InterSystems.Data.CacheClient.CacheFactory, Intersystems.Data.CacheClient, Version=2.0.0.2, Culture=neutral, PublicKeyToken=ad350a26c4a4447c" />
    </DbProviderFactories>
    </system.data>
 

I normally find this in the .NET 2 machine config - and copy it into the other versions as needed.