Hi there. Sorry for delay with answer.
Check these possible solutions and please report which one helped you
1. https://support.apple.com/kb/PH25088?locale=en_US
2. In the terminal, type:
xattr -r -d com.apple.quarantine "/path/to/Rexuiz.app"
maybe will work with leading sudo
Instead of typing the path to the .app, you can just drag and drop the .app from finder into the terminal
3.
a. put the Rexuiz folder in your Application folder.
b. right click on Rexuiz.app
c. Click show package content
d. go in Contents -> MacOS
e. Open a terminal window
f. drag and drop rexuiz-dprm-sdl in the terminal
g. type enter
h. the game should launch without error
4. Here is another bunch of possible solutions from other resource
In the Finder, control-click the app icon, then choose Open from the shortcut menu. Click Open on the dialog.
OR
Use xattr -r -d com.apple.quarantine "/path/to/Rexuiz.app"
OR
Move Rexuiz.app somewhere else and back using the Finder.
P.S, We have very low testers amount with MacOS