If you want to install TFS add-ins, such as Attrice Team Foundation Sidekicks and Teamplain, on the TFS Orcas release you have to install the TFS V8 Client (the one which is shipped with Team Foundation Server 2005). If you don’t have this version both products fails to start/install as they claim they cannot find the correct assemblies.

Once you have installed the V8 client (as well as the Orcas V9 ones) both products work fine with Orcas.