In order to call a C external function (you cannot call a C++ function without declaring a C interface) you must create a PL/SQL gateway function, procedure, or package which references the library.
The syntax for the declaring the library and defining the gateway PL/SQL is too complicated to go over here. For Oracle 9i check out the Application Developers Guide - Fundamentals Chap. 10. I believe this information was in the PL/SQL Developers Guide for previous releases.