I have a mouse but I do not have alt on my mac. On Blender
5$\begingroup$ I'd say this is a Google search type of question that will be faster than waiting for a Blender stackexchange response. And I for one don't know... $\endgroup$– james_tApr 28, 2021 at 18:34
3$\begingroup$ I believe the Mac Option key is equivalent to the Windows Alt key but not having a Mac I can't check. See the answer half way down this page. $\endgroup$– John EasonApr 28, 2021 at 18:51
When Windows people say Alt, use the Option key. When they say left-click, they just mean click. So alt-left click is Option-click.