Ah, thanks DJBMASTER!

After adding the hInstance and some other changes, I got a working example:

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





#define LPWSTR char*
#define MAKEINTRESOURCEW(i) (LPWSTR)((DWORD)((WORD)(i)))
#define MAKEINTRESOURCE MAKEINTRESOURCEW
#define IDI_WINLOGO MAKEINTRESOURCE(32517)


LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);

HWND hwnd1;

char cname[4] = {0x62, 0x6C, 0x61, 0}; // "bla"


void main ()
{
	
	wait(1);
	
	WNDCLASS wc;
	wc.style = CS_HREDRAW | CS_VREDRAW;
	wc.lpfnWndProc = WndProc;
	wc.cbClsExtra = 0;
	wc.cbWndExtra = 0;
	wc.hInstance = hInstance;
	wc.hIcon = LoadIcon (NULL, IDI_WINLOGO);
	wc.hCursor = LoadCursor (NULL, IDC_ARROW);
	wc.hbrBackground = (HBRUSH) GetStockObject (LTGRAY_BRUSH);
	wc.lpszMenuName = NULL;
	wc.lpszClassName = cname;
	RegisterClass (&wc);
	
	hwnd1 = CreateWindowEx
	(
	0,
	cname,
	"Your Subwindow",
	WS_OVERLAPPEDWINDOW | WS_POPUP,
	400, 400, 400, 400,
	hWnd,
	0,
	hInstance,
	0
	);

	ShowWindow   (hwnd1, SW_SHOWNORMAL);
	
	MSG msg;
	while (GetMessage (&msg, NULL, 0, 0))
	{
		TranslateMessage (&msg);
		DispatchMessage (&msg);
		wait(1);
	}
}






LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
	switch (message)
	{
		
		case WM_LBUTTONDOWN:
		MessageBox(0,"Left mouse button pressed inside your subwindow!","!!!",MB_OK);
		return 0; 
	}

	return DefWindowProc (hwnd, message, wParam, lParam);
}