Pearson Education (of Addison-Wesley fame) very nicely recorded a couple of video casts regarding MGrammar recently, and you can find them here:
1. "Oslo" Modeling SDK, "M Grammar" -- What is it? (video)
2. Getting Started with "M Grammar" (video)
Both are informal interviews with Giovanni Della-Libera, the DevLead for MGrammar. In the first interview, Gio provides insight into what MGrammar is, how it fits in with Oslo, and its key features. In the second, he offers some ideas for getting started building your own textual DSLs using MGrammar.