Modern versions are available on the App Store and support the latest versions of macOS and Windows.
Since the developers ( Zerodebug ) moved to a subscription or paid model for the newer versions, older standalone versions are often shared in .rar format on forums.
Unlike the older "midimux.rar" days, modern versions often use the Ableton Link protocol for even tighter synchronization.
The software operated as a pair: an app on the iOS device and a "server" component on the desktop.
If you are looking for this file to fix a specific connection issue or to use an old iPad as a synth, are you trying to connect to a or Mac machine? Knowing your OS can help determine if you need a specific legacy driver.