FYI,
I'm using Microsoft Visual C++ to develop some C code to run as an external
function under the IBM APL2 context in Windows.
Successful debugging can be acheived as follows:
(1) In MSVC project settings, set the "executable for debug session" to
"\ibmapl2w\bin\apl2win.exe". Also set "Program arguments" to "-sm off". You
may also need to set the "working directory" to "\ibmapl2w\bin".
(2) Start debugging (key F5), and the interpreter/command window comes up
(but maybe with no APL character support).
(3) Establish the external function linkage, with quadNA. (see return value
of 1)
(4) Place breakpoints in C code.
(5) Call the external function; MSVC encounters break points at which time
you can examine variables, step, etc.

I got around the lack of APL character support in the command window, by
first specifying the latent expression (quadLX) to extablish the external
function linkage, and perhaps invoke the function as well.

This answers my own question from an earlier (expired) posting.
-Aj