JAVA - SSL - Client Certifcates

Question

I've been developing a WS client using JAVA and I'm having a problem with SSL authentication. The WS are created on WCF and I have no access to the server, they work through HTTPS and uses a client certificate that needs to be installed on the client first. The server guys sent me a PFX certificate which I successfully installed on the OS (I'm using OS X) and I could then access the WS via a browser (Safari or FF are both that I tried which previously couldn't access the WSs). I thought any app in the OS would use this certs but when I'm tried my JAVA app it didn't work; at first the following error was being thrown:

"javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target"

I solved this by exporting the certificate to a CER file and using the keytool command line tool to add the certificate into the "cacerts" keyStore JAVA uses. But after this error went away the following started appearing: "403, forbidden". This is obviously because it's not using the SSL client cert for the site but I haven't been able to find a way to send it to it. Any help would be appreciated.

The following is the code I use to post to the WS:

URL url = new URL(p_url);

HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();
conn.setDoOutput(true);

conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", contentType);

OutputStream out = conn.getOutputStream(); // on this line it shows the error