I have configured developer environment for OpenProj. This works for version 1.4, but I suppose this will work for any release.

Download both bin and -src version and unpack them:
.../openproj/openproj-1.4-src
.../openproj/openproj-1.4

In Eclipse:
0. Open Window-Prefs and go to Java - Build Path - Class Path Vars
and add JRE32_LIB_HOME with path to your jre lib: my path is '/usr/java/jdk5/jre/lib'
1. import existing project: openproj_ui from src. This will display set of errors.
2. import openproj_core
3. import openproj_reports
4. open "Java Build Path" - "Libraries" for openproj_core and edit missed libs by pointing them to jars from openproj-1.4/lib
Make this for both openproj-contrib.jar and openproj-script.jar
5. make the same fix for openproj_reports. Fix error with openproj_reports.jar
6. make the same fix for openproj_ui. Fix openproj-exchange.jar
7. for openproj_ui open build properties and tab "Projects". You will see missing project openproj_exchange. Remove it.

Now you can try to build system. But you will still see some errors for openproj_reports and openproj_core.

8. Add External JAR for openproj_reports - add openproj-contrub.jar
Now try to build openproj_reports. It should be ok.
9. Add openproj-reports.jar and openproj-script.jar to openproj_ui

Now eclipse should show no errors and you can start development.
After your changes added start ant in openproj_build (use command line). This will make openproj.jar in openproj_build/dist. Copy it to openproj-1.4 replacing existing one and start OpenProj with sh or bat file to test your changes.

Let me know if you have any questions.