Under Windows, did you try to "open Run Dialog in Eclipse,
select the Arguments tab of the target application, and add to the VM arguments
text field the option -Djava.ext.dirs=lib;lib/windows" as mentioned in the README.TXT file available at project root and at http://sweethome3d.cvs.sourceforge.net/viewvc/*checkout*/sweethome3d/SweetHome3D/README.TXT?revision=1.15 ?

Under Ubuntu, I guess you used Java 1.4 and not Java 1.5 compiler from the errors you reported ( see the mention "since 1.5" at http://java.sun.com/j2se/1.5.0/docs/api/javax/swing/JEditorPane.html#W3C_LENGTH_UNITS ).