FreeMind development resources
Apart from the documentation pointed to from above, you can learn a lot from FreeMind's development map.
Freemind development map viewed in FreeMind applet; requires Java1.4.
Freemind development map for download
Freemind development map in folding HTML (is reported to crash Mozilla on Linux. This is probably because of extensive use of layers.)
To get API documentation, see generated JavaDoc of FreeMind.