To give you the best advice on that specific file ( Reunion-0.55-mac.zip ), could you let me know: What are you running? Are you trying to open old data from a vintage Mac?
Knowing these details will help me tell you if that version will run or if you need a bridge to the current version. Reunion-0.55-mac.zip
Reunion is a premier genealogy application designed exclusively for macOS, known for helping users organize, research, and visualize their family history. The current version is 14, released as a 64-bit app that supports modern macOS systems and is able to open legacy data files from older versions, including those dating back to the 1990s. To give you the best advice on that
Features an interactive "tree view" that makes it easy to visualize and navigate through generations. If you are looking at an older version (0
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.