WorldPainter needs some dependencies that are not in public Maven repos and cannot be distributed on WorldPainter's private repo due to their licence. You need to install these dependencies into your local Maven repo manually:
For the docks, WorldPainter uses the JIDE Docking Framework, which is a commercial product. For development, you can download an evaluation version of the product here, with user ID and password documented here (note that you need to create a forum account to access the second link). The evaluation version will expire after two months, but you can keep downloading it again whenever it expires for two more months of development time.
Once you have your copy, install the jide-common.jar, jide-dock.jar and jide-plaf-jdk7.jar files in your local Maven repository. Note: if you downloaded a different version than 3.7.13 you must update the version numbers in the pom.xml of the WPGUI module!
To integrate with Mac OS X on Java 8, the Apple Java Extensions are needed. They can currently be found here. Download the .zip file, extract the .jar file from it and install it in your local repo.
Of course if you are not interested in running the code on Apple Mac OS X, or running it on Java 8, you can also just remove this dependency and the code that uses it. WorldPainter will still run on Mac OS X on Java 8, but it will be less well integrated into the menus.
WorldPainter uses the Maven toolchain framework to find the JDKs it needs. You need to follow the instructions on that page to configure two toolchains: one of type jdk and version 1.8 pointing to a Java 8 JDK, and one of type jdk and version 9 (or higher) pointing to a Java 9 (or higher) JDK. Note that it has not been tested whether WorldPainter will run correctly on older Java versions if you substitute a newer JDK for version 9.
Once all dependencies are installed and the toolchains set up you can build WorldPainter from the command line or using your favourite IDE by invoking the install goal on the WorldPainter module. There are some rudimentary tests, but they take a while to run and don't contribute much, so I recommend skipping them by adding -DskipTests=true.
Once it is built, you can run WorldPainter by invoking the exec:exec goal on the WPGUI module, or by running the main class: org.pepsoft.worldpainter.Main.
For a few pointers, pitfalls and gotchas about developing WorldPainter, see this page.
For a more detailed description of the build process, see: https://www.worldpainter.net/doc/building.