There's some info on this page about it, but here it is for your convenience:
Network Play: You can play with another player over a LAN network or over the Internet. For play over the internet, one of the players will need to set up port forwarding (e.g. see https://www.noip.com/support/knowledgebase/general-port-forwarding-guide/ ) for port 17432. This is technical, but fairly common for multiplayer games. If you don't understand port forwarding or don't want to do it, then please don't purchase the full game expecting multiplayer play. There is no matchmaking service for A Kingdom for Keflings on Windows.