One minor thing to mention is that R20 also uses the project tool, so the explanation provided by Kent in part 1 does also apply for that version.
For those wondering, R19 and earlier versions don't use the project tool, but require a complete different setup. Something which is quite a laborious thing to do ... and this for every plugin.
However, I can understand that to some it might seem as quite an effort to get the environment downloaded, installed, and set up ... only to write a plugin.
One tip for the brave ones wanting to develop for multiple versions of Cinema4D: