Attempting to run Cubase 5 on a modern Mac is an exercise in frustration due to the 32-bit architecture limit ("The Great 32-bit Purge" of macOS Catalina).
: For the full base installer, Steinberg provides direct ISO links in their forums and help center. A verified version of the Cubase AI 5.1.1 ISO is available through their official support server. cubase 5 download mac os verified