I tried
java -jar SweetHome3D -Djava.ext.dirs=lib:lib/linux/i386

I'm pretty sure this command can't run, because the -jar option must be followed by a Jar file and the -D option must be cited before the -jar option.
By the way, the README.TXT file states that you should set the directory in the the -Djava.ext.dirs option according to your system and its architecture, i.e. for Windows 7 32 bit, it should be -Djava.ext.dirs=lib/windows/i386;lib and not -Djava.ext.dirs=lib:lib/linux/i386.