Hey Ghostman,
The way I do it is simply put at the top of your file:
#include <test.c>
Then you have to go into your project options and add the location of that file to your 'include search path'.
Project > Build Options > Project > (tab) Directories > (drop-down menu) Include Search Path > Add New (then point it to the directory which has the file you want to include).
^ The menu location is off the top of my head and my memory is sketchy, at best.
but its given me plenty time to play with pics!! so finaly feels like ive had a chance to try and nail a few basics and try a few things out!!! i am finaly starting to be able to get information out the DS!!! YAY (yes its taken way too long).