As far as I recall the default settings for the SIM900 is full Modem control (RTS/CTS) signals besides the usual RX/TX. The MSP430 doesnt support full Modem controls, so you need to emulate them with GPIO.
The RX & TX pins operate from 0V to 2.8V, whereas you've indicated that you're powering the MSP430 at 3.3V, so a level shifter might be needed or series resistors so as not to damage the module.
The communications uses 8-Bit Data, no parity, and 1 stop bit, so make sure the USCI is set up this way.
The Module powers up you need to send either an "A" or "AT" character to get a return of "OK" at this baud rate. The module explicitly supports 1200, 2400, 4800, 9600, 19200, 38400 and 57600, since these are the only Auto Bauding rates (Module powers up in Auto Baud). If you're using the MSP430 'DCO' to create the clock for the UART you may well find that it's out of tolerance for the communications. RS232 comms requires a clock that is better than +/-3%, and the DCO in the MSP430 is only +/-16% in many cases. You might need a proper Crystal connected to XT2 (or XT1 on the F/G2xxx) and use this reference to generate the Baud rate clock for the UART.
You can use the AT command "AT+IPR=x" to set the Buad rate to another setting, but this new value is not stored in the module, so if the module powers down, you have to start over again.
You need to connect to the Serial Port, no the Debug port, since the module has two serial ports.