When opening a Nintex Form with a Lookup control, which allows multiple values from a Mac OS, it will render the Lookup control differently compared to the Windows OS.
On the Windows OS, it will have two boxes with an arrow selection to choose multiple values and from the left box to right box.
The Mac OS devices will render the Lookup control with only one box without any arrows to select the value and it will be moved from left side box to the right side box.  

Choose the multiple values in the lookup control by click Command + Click the value. 

The MacOs has a different way to render List Lookup control with multiple allow values allowed.