According to online sources .ms10 and .mp9 files are projects of different version from NI Multisim software. The software can be certainly used to export both of these files, however it does not offer backward compatibility, meaning it's not possible to create older version of newer version of the projects, i.e. ms10 to mp9 conversion is most likely not possible.
You can of course try to contact NI Multisiim support or some online community for help, perhaps something changed in latest versions.