SSL Certificate Error: certificate_unknown

It is unusual to load the private key as a client request to an SSL socket. I personally think you should be loading the public key. The private key is loaded by the SSL socket on the server side. The only reason you would need the private key is for acting like a server and accepting connections from the other end, but you don't seem to be doing that.

This might explain the "cert in wrong format" message.

In reference to:

private static String certificate = "D:/./trunk/Development/JavaPNS/src/com/applicationname/pns/privateKey.p12"

I think the error is pretty self-evident, your certificate chain does not actually verify your certificate properly. Perhaps you are missing one or more of the chaining certs needed to verify your final end cert?

What ever it is, the error gives you the very clear instruction to "Verify that the keystore you provided was produced according to specs...".

There is nothing anyone on this site can possible add other than to read what your error message says and to do it - none of us have crystal balls, nor can we magically fix your problem.

I don't see the point in posting a question where you give us no more information other than a VERY clear error message that tells you exactly what to do. All we can say is "do what the error message says!"