My client had a preview of the MSMQ/C adapter and was using it in their development environment.  When the new MSMQ adapter came out they installed that and promptly received the following error:

The Messaging Engine failed to create the receive adapter "MSMQ". Reason: "Could not load type Microsoft.BizTalk.MsmqAdapter.MsmqReceiver, Microsoft.BizTalk.Adapter.MSMQ.MsmqAdapter,

We were able to fix the problem by following the steps below

1. Open the Adm_Adapter table in BizTalkMgmtDB database that Biztalk is pointing to
2. In the Name column, find the row with value of “MSMQ”
3. Delete the value in the “InboundAssemblyPath” column
4. Delete the value in the “OutboundAssemblyPath” column as well
5. Restart the BizTalk host

After following these steps we did the MSMQ adapter install and all is working.