If you need to quickly connect to a device that uses DHCP to make some changes...
In the OSX System Preferences > Sharing,
Enable Internet Sharing to computers using Ethernet,
Then open a terminal, and run:
tail -f /var/log/system.log
Connect the device via Ethernet, and wait for it to power up.
You should see some output on the terminal, such as:
DHCP REQUEST [bridge0]: 1,aa:aa:aa:aa:aa:aa <raspberrypi>DHCP DISCOVER [bridge0]: 1,aa:aa:aa:aa:aa:aa <raspberrypi>OFFER sent <no hostname> 192.168.4.5 pktsize 300ACK sent <no hostname> 192.168.4.5 pktsize 300
This gives you the IP address of the device.
However you may not be able to ping it, as it could be on a "bridge" interface:
ifconfig bridge0: inet 192.168.4.1ping -b bridge0 192.168.4.5 PING 192.168.4.5 (192.168.4.5): 56 data bytes 64 bytes from 192.168.4.5...
To add the route to this network, just call:
sudo route add -net 192.168.4 -interface bridge0
And now you can connect to it via SSH, or other protocols.