I got word back from GS support. ScanMessage was not setup for Lite-C, only for DLLs. This will be fixed in the next update along with an example in the manual. Until then, you'll need to edit avars.h like this:
Code:

---

#ifndef acknex_h //if using DLL
		void  (*SendPacket)(long to,void *data,long size,long flags);		// the send function of the engine
		void  (*ReceivePacket)(long from,void *data,long size);				// user provided receive function
		long  (*ScanMessage)(UINT message, WPARAM w_param, LPARAM l_param);	// user provided message handler
#else // use normal pointers for lite-C
ENGINE_ void *SendPacket;
ENGINE_ void *ReceivePacket;
ENGINE_ void *ScanMessage;
#endif

---

and a working complete example of using it (resize the window and it will beep, all the defaults still work):
Code:

---

#include <acknex.h>
#include <windows.h>
#include <default.c>

function MyMessageFunction(UINT message, WPARAM wParam, LPARAM lParam);

function main() {
	ScanMessage=MyMessageFunction;
}

function MyMessageFunction(UINT message, WPARAM wParam, LPARAM lParam){
	if(message== WM_SIZE){ beep();}	
}

---

Since A7.06

#include <acknex.h>
#include <windows.h>
#include <default.c>

function MyMessageFunction(UINT message, WPARAM wParam, LPARAM lParam);

function main() {
on_scanmessage = MyMessageFunction;
}

function MyMessageFunction(UINT message, WPARAM wParam, LPARAM lParam){
if(message== WM_SIZE){ beep();}
}

yup, this thread was started before 7.06. This issue is now well documented.