I've updated the page for MPW AsmIIgs Formats with a link to a mirror of MPW, since Apple has removed it from their site. In addition, I've added some more information about specifically which files you need to download and a bit of information about the format of the image file you're downloading.
In addition, I've added information about Kelvin Sherlock's awesome mpw command for Mac OS X, which lets you run MPW commands directly from the Mac OS X terminal, instead of having to use a classic Mac or emulator to do so. This means you don't need my MPW editor plugin at all, of course, since you'd be using a Mac OS X native editor instead.
Hopefully that's helpful!