OK. For those viewing this post for a solution, I figured it out.
Even though I had RealPlayer 8 installed correctly and the shared object rpnp.so is the correct plugin, I had the wrong entry in the Mozilla Download manager. When the manager comes up, you should click the 'Open With' radio button and select /usr/local/RealPlayer8/realplay in the program text box. Once I did this, Mozilla launched real player which opened the file.
Thanks chem1 for the reply. Looks like you dont need raclass.zip after all though.