Knowing these details will help me tell you if that version will run or if you need a bridge to the current version.
It allows for comprehensive data storage, including names, dates, places, notes, media (photos, documents), and research logs. Reunion-0.55-mac.zip
If you are looking at an older version (0.55), it likely predates Mac OS X or is from the early days of it. For modern usage and maximum stability, the current Reunion 14 is recommended. Knowing these details will help me tell you
Reunion 14 for the Mac - Digital Download - S&N Genealogy Supplies Reunion-0.55-mac.zip
Features an interactive "tree view" that makes it easy to visualize and navigate through generations.