Regarding the error "damaged and can't be opened. You should move it to the Trash.", issue 590's xattr -rd com.apple.quarantine Google\ Refine.app/ worked for me.

Thanks to this for pointing out that extensions are installed on the Mac at ~/Library/Application Support/Google/Refine/extensions/. According to extensions, you can also get to it by clicking "Browse workspace directory" at the bottom of the page at https://localhost:3333.