To debug you function create a report or application that uses the function, the with visual studio attach to the activ console (fat client) process, place break points and run it.
As for NER. It is possible to code it using NER but you will have limitations, no cache operations, no pointers, no arrays, you get the picture
Finally, please include in you signature you sistem configuration so other more specific questions will be figured out more easily.
You should be able to find many documents about how to attach a C session to E1. Instead of attaching to E1 you can attach to OW Info and test/debug using the Run window without the need to create an application with all the input and output parameters. The executable to attach to will be OW7.exe or OW8.exe depending on your release.
NER functions can do many of the things that a C function can do; table IO's, function calls, etc.
However, you can't directly use caches, arrays and other more advanced techniques.