If you’re getting this message, it means that you are using a version of MultiMarkdown Composer earlier than v4, which is currently in beta and (hopefully) nearing completion.
More information about v4 is here. Other pages on that site contain additional discussions from the past year or two about the new versions of Composer if you haven’t read them.
If you’re using the beta of v3, I recommend that you consider upgrading. Development efforts on v3 have now officially stopped, and all work is being done on v4.
If you’re using v2 from the App Store, you’re welcome to either one:
v2 has been around for a while, and is very stable
v4 is newer, but much faster and has more features. It’s stable, but there is still the possibility of undiscovered bugs that need fixing
Download v4 from the link above and try it out.