You are not doing it correctly. The correct syntax is "ln -s <location of the original file> <location for the target/link>. Basically, it should be something like this:
ln -s /usr/java/jre1.5.0/plugin/i386/ns7/libjavaplugin_oji.so /home/<your user>/.mozilla/plugins/libjavaplugin_oji.so
Note: the above won't work. You need to link your java plugin to your plugins directory in mozilla directory at your home.