Is it possible to play TA multiplayer via internet?
I tried to do so but it didn't seem to work. Searched by IP etc. but couldn't find each others hosted game. Firewall is turned off and OS: Mountain Lion vs. Lion.
What is blocking TA and how can this be fixed?

I have the same problem. By screencasting the connnection try, we spotted TA trying to update network listing but failing. It fails no matter what the network settings.

I've had no trouble playing TA multiplayer on my local network between two macs. The game browser screen doesn't identify the computer that is set up to host, but by manually typing in the IP address of the host box the client is able to connect. Over the internet should work the same, though you'll have to put both server and client on the DMZ of your router. You won't be able to play more than 2 player without a VPN or a local network.