General developer forum

It gets set initially in lib/defaults.php with a default value.

Later it can be changed on Admin -> Configuration -> Enrolment page which is not in CVS yet.

Plugins all work slightly differently, but basically you just add a directory with code in it to:

mod
auth
enrol
filter
lang
blocks
course/format
resource/type (new!)
quiz/question (new!)
quiz/format
quiz/report

Moodle scans these directories when appropriate to pick up new modules in there.