How do i play mp?

[Warhound]

Click multiplayer in the menu, then client, pick a server. Then the map loads which can take a while.
Once done you can pick your side in the top left (click "select army"), then select one of the white airbases and a menu will appear where you can choose an aircraft. Pick your plane of choice and it's fuel/bomb loadout. Then it's just a matter of clicking "create" at the bottom right and you will spawn ingame.