hello there ! first, i’m happy to join the rhino mac community and i love the interface… grasshopper is still missing but mac is not windows I know… so any way great work !!!, for the question, it’s silly for sure… but i have tried almost everything i guess and that begins to make me crazy 
for a such simple stuff it’s just no sens … so here you are 
Rhino uses the standard Windows selection keyboard conventions.
Holding Ctrl and selecting a previously selected object will remove it from the selection.
Holding Shift will add objects to an existing selection.
Hello John as expected of course, but maybe only on my mac ctrl+clic (from track pad) make me pop a menu … 
on mac:
shift click to add to existing selection.
command click to deselect.
Perfect Thanks !