Brownout (low voltage) Detection on msp430F2274
From MSP430x2xx Family User's Guide (slau144):


  The brownout reset circuit detects low supply voltages such as when a supply voltage is
  applied to or removed from the VCC terminal.  The brownout reset circuit resets the device by triggering a POR signal when power is applied or removed.  The operating levels are shown in Figure 2-2.


This suggests that the brownout detection signal is not available to the firmware.  If you want to detect low battery you need a different means.  Probably some sort of external circuit triggering an interrupt when battery voltage drops to just above the "brownout" level.