This message means that your system tried to run Sweet Home 3D with a version of Java that is too old (Sweet home 3D requires Java 1.5 at minimum). Under Mac OS X, ensure that the SweetHome3D.jnlp downloaded file is opened by Java Cache Viewer (11.5.0) application. Under Windows, you should install the latest version of Java available at http://java.com then click again on the link Launch Sweet Home 3D with Java Web Start.