Just by giving the datasheet a quick glance I noted that it uses SPI to communicate. Have you configured SPI on your microcontroller? This would be the first step. After you verify that SPI is configured correctly and that you can send out a byte onto the SPI bus, then plug in the RF module and try again. This allows us to eliminate SPI as the origin of your problem. After that, I would suggest looking at the setting of the RF module and verifying that they are configured for SPI communication and that they match what you are trying to send out via SPI.
An application note from Atmel that provides multiple (and very well commented) code example on how to configure SPI on the 8051... http://www.atmel.com/Images/doc4348.pdf
Once you have browsed the articles and have gained a better understanding of SPI, start writing code to send out a byte via SPI; use the Atmel application note for help. Once you have done this, we will be able to move on and try to communicate via the RF PMOD. If you run into trouble while programming the SPI, post your code and we will be more than happy to help.