You're like a mistery novel: always reveal bits and pieces of the puzzle one at a time
I'm not entirely sure I grasp the situation though. You don't have the code or IDE to test/debug the .Net software, so you purchased it. Is there no support from the supplier of the software?
And if the supplier also delivered the database the software seems to use, my guess is you don't have access to the stored procedures unless maybe execution rights. If that is the case I doubt you'll ever be able to debug this problem yourself unless you're in the lucky situation where there's ample logging of what's happening on the Oracle level of the application.
If you do have access to the PL/SQL code I doubt any of us can do more than point you to the brief howto I wrote earlier in this thread or mention the manual that comes with PL/SQL Developer.