First and foremost, don't use Cocktail on the server as-is. Many parts of Cocktail are not threadsafe and that includes the single shared MEF container. Cocktail is a client framework written with a single UI thread in mind. A server is a different animal all together starting with the fact that it is multi-threaded by design.
The above won't work, because the WCF service is not created by MEF. You could call Composition.BuildUp(this) in the constructor to satisfy the imports, but again, don't do this on the server with Cocktial as-is, unless you want to create yourself a headache down the road.
If you only want to use the Compositon part of Cocktail on the server, the way to do that is to implement a threadsafe version of ICompositionProvider that sets up the MEF container per thread instead of a globally shared container. You can then call Compositon.SetProvider to configure Cocktail to use your threadsafe composition provider.