1. After you get the AU validation error alert, go to System Preferences: Security & Privacy: General tab
(There may be a padlock icon in the lower-left, click on it to authenticate and unlock it)
2. Click on 'Allow Anyway'
3. Return to Logic Pro X, start the Plug-In Manager
4. Find Ribs, and click in the Use box, it should now be checked:
5. Click on 'Reset & Rescan Selection'
6. You may get an alert along the lines of:
Obviously, click on Open.
You maay see an Audio Unit Validation Result window:
And Plug-In Manager also show successfully validated:
Good luck!