Confirming the same behavior on OS X 10.10.3: Failed to find data folder. I happen to also have a copy of the GOG version installed, I used my second backer key to install on Steam to try running the beta. I compared to contents of the bundles and identified the missing files: in Data files matching level*, *assets*, and the folders Managed and Resources were missing.
After copying those over the game launches from Steam, but it claims to be 1.04 on the title screen in the lower left corner. I'm not sure if the game will behave as 1.04, 1.05b, or a mixture from behavior to behavior with this setup, and I'm not really planning on testing it. Just letting you know what files were missing from the 1.05b push.