You make an MC6 file very simply, by saving into the materials library (which is only visible while you're in the material room, by the way). Choose "material collection" to get an MC6 which includes all the material settings for the thing you have selected at the time, or "single material" to get an MT5 which is just the one material you have selected.

Edited to add: MT5 / MC6 files have to live in the materials library.

Assuming you're running some version of Windows, you probably have extensions hidden which makes it difficult to change them. In Windows Explorer, go to Tools > Folder Options > View (this is for XP - may be different in other Windowses). Then uncheck "Hide extensions for known file types". After you've done that, you can rename the file in the usual way, by single clicking on it twice - or right click and select rename.