ln -s /usr/java/j2re1.4.2_03/plugin/i386/ns610-gcc32/libjavaplugin_oji.so
Restart netscape and go to 'Help -> About Plug-Ins' and you should see it listed. Then go to any Java web page and it should work.
If 'j2re1.4.2_03' isn't your java version, just replace it with the correct one. The easiest thing to do is after typing 'ln -s /usr/java/' just type 'j' and press tab. It should auto-fill the correct dir name in.