Launch the game on steam, the windows version, and it will say "invalid platform". Don't close that popup saying "invalid platform". But now when you open Muck from the mac/linux executable and try hosting it will work.
This sounds weird, but launch the game on steam even though it is Windows, it'll say "invalid platform" in a popup, don't close that popup and now launch the mac/linux executable and it should work.
To anyone having hosting/launching trouble on Linux:
chmod +x "executable name here"
And on Steam launch the game from the Windows store page, it'll say "Invalid platform", but keep that dialog open and launch Muck from the extracted path and now you'll be able to host games.
