If the catalog you have was created with an older version of Lightroom (older than version 5) then Lightroom 5 installed from the creative cloud will prompt you to allow it to convert the catalog to the format that is compatible with Lightroom 5. After the conversion takes place you will have a new copy of the old catalog along with all the adjustments that were made in the previous version.
But it puzzles me that you say you weren't able to open it on the updated Mac. Why were you not able to do that? Were there any error messages? If the catalog is damaged and Lightroom cannot convert it then it will be necessary for you to start over.
No error messages. It puzzles me, too.
Thanks, though, I'll try.