'Let's play Heroes 5 online.'

First, download hamachi (it must be installed on both your computer and your opponent's).
You don't necessarily need to download a patch. However, your game version and your opponent's game version must be the same.
Launch hamachi. Create a new network. Your opponent joins hamachi and clicks "Join existing network," selects the network you created. If everything works correctly, a local network (its emulation) will be established between you.
Next, go to Heroes 5 – Local Network, select the game mode, then one player creates a map (arena), and the second player joins. That's how it worked when I played using hamachi.