A previous client of mine ran into this problem while installing the MSMQ adapter during a production push.  They were logged into the server using a Host instance login, which happened to be a local administrator.

The minimum security requirements for adding an adapter is that the user has to be in two BizTalk Server domain groups, the BizTalk Server Administrator and the SSO Affiliate Administrators groups.

In this case, the user was not in the SSO Affiliate Administrators group.  Logging in as a user that is in both of the aforementioned groups resolved the issue.