I suppose that you want to call a C++ Method from a CORBA class written in Java, right? If yes, the aswer is simple.. you must to use a ORB wich have C++ Mapping (MICO, www.mico.org, is one of these - and it's opensource), and you must to write an appropriate IDL file wich contain your method, compile it in way to obtain C++ wrapper classes, implement your method, ad so on :) If you want, you can download a free orb (for non commercial use, if i remember correctly ;-) called Engine Room CORBA (www.engroom.com), wich have both Java and C++ IDL mapping, in this way you can do very cross-language CORBA experience ;-)
Regards.