Hi, I already did this and this doesn't fix it. I followed the exact same procedure to run Pokemon Reborn on my Mac, which works fine.
I have the option 'Allow apps from Apple and identified developers' selected. When I open Rejuvenation (in Applications), I then get a warning message that it's not identified by Apple, so I went to the Security preferences and manually allowed opening of it by clicking on 'Open anyway' and get the error message I posted a while back.
I guess that there's some scripting issue and we'll have to wait a bit. Just asking, did you start a new game, or did you use an old save file?
I was going to try experimenting with manually making a folder for Rejuvenation in my hard drive folder and put an old save file there. EDIT: This didn't work, and I didn't expect it to, but had to try :(
Note: I tried the download link on the Mega server and the Reborn mirror. No luck with either.