Migration Assistant (MA) in Source remain "looking for other computer" state forever
Can you ping one Mac from another? Look in the network control panel and find the IP address of each Mac. they should be similar. Like 192.168.01 and 192.168.0.2 where the IP address of each is only different in the last number. If that is not the case you can try assigning each a unique IP address manually and try again. failing that...

Can you plug the computers directly in to each other with a plain Ethernet cable and reboot both while connected and then try again? The network control panel in that case should show both with a 169.x.x.x IP address that is different than the other Mac. If that is not happening assign each a manual IP address in the range of 192.168.0.x, where X is a number between 1 and 254 and different on each Mac 

The other thing to try is to put the old system into target disk mode (Command-T while booting) and then connect the two with Thunderbolt or Firewire cables and run migration assistant in LOCAL mode. EG select the option to copy from a locally attached disk.

Another way would be to image the old Mac with Carbon Copy Cloner, SuperDuper or even Disk Utility to an external (USB or other) drive and then plug that into the new Mac and run Migration Assistant in local mode as above.

Exactly what is going on or why, I am unsure but those alternatives should help you get up and running.