OK, this means MEF is unable to find or load a plugin for the given deliverymethod. If you're using adaptive deliverymethod, you just need to have:
A few more things to check:
Make sure all of these assemblies are in your main Silverlight project. Do not rely on dependency project refereneces to include these assemblies in your xap
Make sure Copy Local = true for all project references (not sure if this is necessary)
Make sure that the assemblies are "unblocked". This can happen if you download a zip file from the web and don't unblock it before extracting. Right click on each dll in Windows Explorer and check for an unblock button.
Make sure Microsoft.Web.Media.SmoothStreaming is from the v1.5 Update 1. The dll should be version 1.5.938.92. There is a link to the setup on our download page.
Last resort: Clean your solution, reload Visual Studio and rebuild your solution.