I bought a new Macbook running Lion, trying to migrate from an old Macbook running Snow Leopard. At my temporary housing, the two machines were able to find each other over migration assistant, but since the network is so unreliable (it took 30 minutes just to connect each machine), I can't use it for migration. At work, we have an excellent wifi, but the two machines can't find each other in the migration assistant (despite being able to ping each other). Any guesses as to why?
My first guess is that your work's network configuration could be blocking Bonjour broadcasts. But in my experience doing a migration over WiFi can be a bit flaky anyway. Try it several times and maybe it will eventually work.
Instead of using WiFi, I would recommend using a wired connection if possible. It will go many times faster. If both computers have an ethernet port you can just connect them directly to each other with an ethernet cable. Or you can connect them via FireWire or Thunderbolt if you have one of those cables.
If only one computer has ethernet, try plugging that computer in via ethernet and connecting to WiFi with the other. That will help the speed and reliability.
Another solution would be to create a new TimeMachine backup on a erased USB external drive and then run Migration Assistant using the TimeMachine option. If you don't have a USB drive, you can get them relatively inexpensively on-line or at a big-box office supply store. Its nice to have one or two spare drives like this in the toolkit. I've done this type of migration several times for myself and others.
Have a glass of wine and make dinner while the backup finishes, then eat dinner and dessert while the migration occurs. No need to hurry things.