Migration Assistant: 10.5.8

My iMac is running Yosemite, and for years I've used Time Machine to back it up.

Just bought a MacBook Pro 5,1 running 10.5.8. Since I'm traveling soon, I'd like to use Migration Assistant to bring the Time Machine data to the MacBook Pro.

Problems:
. When I choose From Time Machine, it never sees my external TM HD. I've tried 3 different USB cables.

. When I choose From My Other Mac > Network, my iMac never discovers the MacBook Pro. Both machines are on my wi-fi network. I've turned wi-fi off/on on both machines. I've turned File Sharing off/on on both machines.

Any insight? Thank you!