When I first downloaded it, I had a the 1.0 version (unstable?) of MCedit which is third on the download page. I'm pretty sure I had that one and it asked to update, so it updated. I downloaded the filter but it didn't show up, it looks like that version needed two versions of the file. Py and something else.
So I deleted that version and downloaded 1.1, first on the list which didn't ask to update. The filter showed up. I made an absolutely massive selection and made it a desert. It worked as usual, but when it finished and the chunks were rendering, MCedit closed and nothing changed. "1 unsaved edit" didn't show up before it crashed, while it should. I think this is an unstable version so it can't handle large selections?