I recently did a fresh setup of my computer and took lots of notes. I figure this info can help some people.
Here you will find everything you need to know about getting Eclipse setup to make Max/MSP externals: http://compusition.com/web/articles/maxmsp-eclipse
Others have already provided instructions on this, but I assume absolutely no Eclipse experience. So if you are a complete newb this should help.
I expect to be doing more of this in the future but hopefully on more interesting topics. I'm still trying to figure out the right format, so any feedback is welcome.