If you’re getting this message, it means that you are using a version of MultiMarkdown Composer earlier than v4, which is now available for sale in the App Store.
More information about v4 is here.
NOTE: version 4.0.1 has been accepted and should be available. Sometimes it takes a while to propagate through all App Store regions outside the US (up to 24 hrs per Apple). I recommend waiting until your App Store indicates version 4.0.1 to avoid some issues in 4.0. Thanks!