Mac Rhino in version 5 includes three different processor architectures. At start time, OS X picks the appropriate version of the program to run. If you have a PowerPC processor, OS X will pick the 32-bit PowerPC version of Mac Rhino. If you have an Intel Core 2 Duo processor or later, OS X will run the 64-bit Intel version of Mac Rhino. If you have a Core Duo Intel processor, you will be running the 32-bit version of Mac Rhino.

The Rhino > About Rhino box will show you which version you are running:

In some situations, you may wish to start Rhino in 32-bit Intel mode rather than 64-bit mode. To do so, select the Rhinoceros 5 program in the Applications folder in the Finder and select File > Get Info in the Finder's menu. A dialog like the following will display:

Quit Rhino if it is running.

Check the box that says “Open in 32-bit mode”

Close the dialog and start Rhino. It will now be running as a 32-bit application.

The About Rhino dialog will now look like this:

You can switch Rhino back to a 64-bit mode by clearing the “Open in 32-bit mode” check box.