Here's the solution.
Go to your blockland folder (library/applicationsupport/steam/steamapps/common/blockland)
There should be a file simply called "blockland" delete it.
Go to the blockland.us website and download the demo for mac. Put it in the folder you just opened. Place it there, and rename it as "Blockland".
So basically, you are just replacing the steam file with a new one.
It works for me, hope it will work for you.