I don't know if this will help you or not, but I had the same problem the first time I tried using PS3MediaServer. No matter what I do, It just won't find my PS3.
Then I tried to change some value in "General Configuration" tab.
I changed "Force IP of the server" setting to my mac's ip address. My mac and PS3 were connected to the same router via Wi-Fi.
I set up my mac ip address to 192.168.1.103 and my PS3 to 192.168.1.105
Then I put my mac ip address ( 192.168.1.103 ) in the "Force IP of the server" field and voila ! the PS3MediaServer finally founds my PS3
dunno if it will works for you but it's worth the try