atmel studio 6 and v-usb

hi
i want to use atmel studio 6 to compile v-usb project.

i created a new project > gcc c executable project
device > atmega8a

and here is my code

#define F_CPU 12000000

#include <avr/io.h>
#include "usbdrv.h"
#include <avr/interrupt.h>
#include <avr/wdt.h>


#include <util/delay.h>
#define USB_LED_OFF 0
#define USB_LED_ON  1

USB_PUBLIC uchar usbFunctionSetup(uchar data[8])
{
    return 0; // should not get here
}


int main() {
   DDRB = 0xff;
   PORTB = 0xff;
   uchar i;

   wdt_enable(WDTO_1S); // enable 1s watchdog timer

   usbInit();
   usbDeviceDisconnect(); // enforce re-enumeration
   for(i = 0; i<250; i++)
   { // wait 500 ms
      wdt_reset(); // keep the watchdog happy
      _delay_ms(2);
   }
usbDeviceConnect();

sei(); // Enable interrupts after re-enumeration

while(1)
{
   wdt_reset(); // keep the watchdog happy
   usbPoll();
}

return 0;
}

when i compile > error :
undefined reference `usbinit();'
undefined reference `usbpoll();'
undefined reference `usbInputBufOffset'
and ....


what is the problem?