I don't know the Mac specifics, but in General.
If you want more than one IDE installed, then
- install them in different path
- specify --primary-config-path= or --pcp=
You seem to have replaced the 2.2.6 install with a 3.0
But one caveat. While the installed executable "lazarus.app" is upgraded, there is a shadow executable that is still 2.2.6.
This happens when you add packages, which rebuilds the IDE. But the newly build IDE can (due to user permissions) not be written in the global install location.
So it ends up in your home folder in the config dir (somewhere in it) ~/.lazarus/
You probably use "StartLazarus" and that finds this old 2.2.6
You can keep the install as it is.
Option 1)
remove that old version, from the config path => then 3.0 should start (you need to rebuild it, to get your packages)
Option 2)
Just rebuild your IDE
The Warning is, because the new and the old ide have that file in a different location....
You can ignore the warning, it will resolve itself with the above steps