How do you use low-power mode on an MSP430G2553 using an Energia sketch?
LPM modes are turned on by setting the appropriate bits in the status register:

_bis_SR_register(LPM1_bits);


Any LPM mode is automatically turned off when the CPU starts running an interrupt handler. (When the handler function returns, LPM is re-entered because the old SR value is restored from the stack.)

Interrupts should be setup in the setup() function.
You can go to sleep at the end of the setup() function, or in the loop() function.