Hiya,
Crossfire is an webbased firefly client, and thus requires a webserver to be installed on your machine ;). OSX does come with apache installed, check http://www.oreilly.com/pub/ct/49 for more information..
After you have learned how to host websites on your server, things should be pretty straightforward.. install PHP, and copy the scripts to the right webfolder.. To connect to your NAS by default, just edit the config.xml file and put in the right hostname.
cheers,
Tony