I get the same error. Not sure if we need to un-install and re-install? Though why would Adobe give me the option to update upon launching the application if that wasn't the way to perform the update?
We are currently facing issues with Mac installer of Muse and our team is working to get it fixed.
You download and install the working installer from the link : http://labsdownload.adobe.com/pub/adobemuse/builds/muse_2-2-1_install_mac.dmg
You can refer to Post # 29 by Colby on forum post : http://forums.adobe.com/message/4761252#4761252
Please let us know the results.
Thank you so much, Sachin! That did the trick.
This worked for me after trashing the existing Muse app and downloading the new updater in Post #29. Thanks.