The datasheet - http://www.parallax.com/Portals/0/Downloads/docs/prod/rf/27982-433MHzRFTransceiver-v1.1.pdf - talks about asynchronous serial @ 1200 – 9600 bps
so you need to try 1200, 2400, 4800 and 9600 baud . The code samples all use 2400 baud so that's the one I would start with.