Applets signed with selfcert are by default not valid for the plug-in.
For testing purposes or some pc's on a small
network you can generate a own CA Key and Certificate (for example with openssl)
and import it to the /jre/lib/security/cacerts keystore.
Then generate a object signing certificate signed with the
own CA key and signing the applet with this cert.

This is a little bit complicated bit works
fine for testing, but if there are many computers
in a large network, you should get a offical
siging Certificate from Verisign or another
CA company.