i can't watch the local varibles in debug mode with ccs v4

hi could you please help me with this issue because a i want to see the varibles 

but ccs tell me that he can't found in the memory the variable, i'm using the msp430g2231.

this the test program. the variable to watch is "a"

#include <stdio.h>

#include <math.h>

#include <stdint.h>

#include  <msp430g2231.h>

unsigned char a;

void main(void) {

 WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT

  P1DIR |= BIT6;

  P1DIR |= BIT0;

  P1DIR &=~BIT3;                            // 

  TACCTL0 = CCIE;                             // CCR0 interrupt enabled

  TACCR0 = 40000;

  TACTL = ID_3 + TASSEL_2 + MC_2 + TAIE;                  // SMCLK, contmode

  _BIS_SR(GIE);                 // ENABLE interrupt

  for(;;){

  if((P1IN & BIT3)==0){

  P1OUT |=BIT6;

  }

  else {P1OUT &= ~BIT6;

   } 

  }

}

// Timer A0 interrupt service routine

#pragma vector=TIMERA0_VECTOR

__interrupt void Timer0_ISR (void){

   P1OUT ^= BIT0;

  a=a+1;

  if(a>10)a=0;                           

}  

thanks for your time and i hope with your help solve it!!!