Here is how to set AI Traffic density from a custom DLL inside FSX

// FS10 FS-TRAFFIC.DLL - Settings Structure
typedef struct FS10TRAFFIC_SETTINGS
{
    unsigned long Traffic_Density; // Traffic Density %(percent, from 0 to 100)
	unsigned long IFR_or_VFR; // 0 = IFR and VFR, 1 = IFR (Only)
	unsigned long Airline; // 0 = Disable, 1 = Enabled
	unsigned long General_Aviation; // 0 = Disable, 1 = Enabled
} FS10TRAFFIC_SETTINGS;


// FS10 - Exported Function Pointers Types Definitions
typedef void  (FSAPI *FS10TRAFFICWRITESETTINGS)(FS10TRAFFIC_SETTINGS* info);
typedef void  (FSAPI *FS10TRAFFICREADSETTINGS)(FS10TRAFFIC_SETTINGS* info);


FS10TRAFFIC_SETTINGS FS10Traffic_Settings;


// Initializate this structure
memset(&FS10Traffic_Settings, 0, sizeof(FS10TRAFFIC_SETTINGS));


// Get the handle of the TRAFFIC.DLL module
hTraffic = GetModuleHandle("FS-TRAFFIC.DLL");


// READ AND CHANGE THE TRAFFIC SETTINGS
// If we got the handle of the TRAFFIC.DLL module then
if (hTraffic != NULL)
{
	// 1) READ THE CURRENT TRAFFIC SETTINGS
	// Get a pointer to the TRAFFIC.DLL module's exported function
	FS10ReadTrafficSettings = (FS10TRAFFICREADSETTINGS)GetProcAddress(hTraffic, (char*)6);

	// If we got a pointer to the TRAFFIC.DLL module's exported function then
	if (FS10ReadTrafficSettings != NULL)
	{
		// Get the *CURRENT* traffic settings
		FS10ReadTrafficSettings(&FS10Traffic_Settings);

		// Notify to user
		MessageBox(hWnd, "The traffic settings had been read SUCCESSFULLY!", "FSIntruder.dll - Message", 0);
	}

	// 2) CHANGE THE TRAFFIC SETTINGS
	// Get a pointer to the TRAFFIC.DLL module's exported function
	FS10WriteTrafficSettings = (FS10TRAFFICWRITESETTINGS)GetProcAddress(hTraffic, (char*)5);

	// If we got a pointer to the TRAFFIC.DLL module's exported function then
	if (FS10WriteTrafficSettings != NULL)
	{
		// Build the new traffic settings structure
		FS10Traffic_Settings.Traffic_Density = 100; // 100% of Traffic density
		FS10Traffic_Settings.IFR_or_VFR = 0; // IFR and VFR
		FS10Traffic_Settings.Airline = TRUE; // Enabled
		FS10Traffic_Settings.General_Aviation = TRUE; // Enabled

		// Set the *NEW* traffic settings
		FS10WriteTrafficSettings(&FS10Traffic_Settings);

		// Notify to user
		MessageBox(hWnd, "The traffic settings had been CHANGED SUCCESSFULLY!", "FSIntruder.dll - Message", 0);
	}
}