Try pinging the broadcast address. This should work as long as the Pi got an IP from DHCP and responds to ping.
First, open up a terminal and run
ifconfig en0. This should give you the info for your ethernet interface, if not, just use
ifconfig and find it yourself. Make a note of the broadcast address. In my case, it's 192.168.2.255:
inet 192.168.2.99 netmask 0xffffff00 broadcast 192.168.2.255
So I'll run the command
ping 192.168.2.255. Do the same with your broadcast address, and hit CTRL-C to stop the ping once you have some results:
Miramar:~ gabriel$ ping 192.168.2.255
PING 192.168.2.255 (192.168.2.255): 56 data bytes
64 bytes from 192.168.2.99: icmp_seq=0 ttl=64 time=0.113 ms
64 bytes from 192.168.2.1: icmp_seq=0 ttl=255 time=3.567 ms
64 bytes from 192.168.2.99: icmp_seq=1 ttl=64 time=0.096 ms
64 bytes from 192.168.2.1: icmp_seq=1 ttl=255 time=3.455 ms
The broadcast is intended to go out to everything on the network. Everything that hears it should respond. In my case, I only see my IP and my router's - I guess it didn't bother passing the request on to my other machines, but you're directly connected to the Pi so you should see it there.