I'm using the IComSat v1.1 with an Uno R3 to send and receive SMS messages.
Communication with it is done with SoftwareSerial. My only complaint is that I can't communicate reliably with baud-rate higher than 19200 (I couldn't figure out why this is the case, and simply gave up. 19200 baud is good enough for me).
I didn't use any of the provided software (nor any other available library) because my system must respond to several external events in real-time, and I had to implement a cooperative multi-tasking system, where delay() is never ever ever used.
What I suggest is that before trying to accomplish complicated things like sending SMS, just use a trivial pass-through sketch and see if you can get the SIM900 to response to your AT commands. This is the "hello world" of Arduino to modem connection. When you'll get this right, you'll know that you've got everything configured correctly.