I finally made it!!!! It works!
in mac osx I go to the firewall menu and enable incoming connections from the application ps3 media server, check! Thank you for that.
in order to connect the media server to the ps3 I need to go to the ps3 network configuration, select manual ip config instead of pppoe (which I use to play online), ip 192.168.1.200 for example, subnet mask 255.255.255.0, router 192.168.1.1, dns by default (we all know them), mtu, no proxy, upnp enabled, and finally X !It doesnt matter if at the end a message with dns error appears because the connection is between the ps3 and the compter with the media server....not between the ps3 and the internet! That's what I didnt understand.
Back to java ps3 media server force the network interface to lo0, force the ip server (the ip of the computer, let's say 192.168.1.128), and it's done!! The big shiny ps3 icon appears and on the XMB the media server works!
If I want to play online I need to go back to the network configuration of my ps3 and select pppoe instead of manual config