How to install language plugins
MPS can be easily extended with additional languages. Languages come packaged as ordinary zip files, which you unzip into the MPS plugin directory and which MPS will load upon restart.
The most convenient way to install language plugins is through the Plugin Manager, which is available in the Settings dialog (Control + Alt + S / Cmd + ,).
You can either install a zip file you've received previously (the Install plugin from disk... option) or you may click the Browse repositories button and pick the desired plugin from the list of plugins that have been uploaded to the MPS plugin repository. 
A handful of language plugins (both by JetBrains and third parties) for MPS as well as for IntelliJ IDEA have been made publicly available at the JetBrains plugin repository:
MPS Languages Repository page in the documentation
MPS language plugins site