If you don't have Java3D installed and part of your classpath, then when you compile MASON with its Java3D extensions, it will naturalliy complain that various Java3D classes (on which those extension depend) are missing. That's what you're seeing in your compile-time exception.

There are two solutions:

0. Simply delete the 3D facility.  This consists of the sim/display3d and sim/portrayal3d directories.  Just delete them.  Then you should be able to build without any problem.

1. Use mason.19.jar instead of the raw code. This has all the class files precompiled (including for Mason's Java3D apps), so you should be able to compile against it without any issue.   And if you *run* a Java3D application from mason.19.jar (for example, HeatBugs3DWithUI) mason will try to load Java3D, fail, and issue a reasonable warning.