The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Update - I am able to get it going with a reinstall, however, as soon as the client is closed and reopened again (with or without playing a game) it will not load the "Play" or "Mods" section, wont play a sound on launch, and will stop responding and log me out.