Right-click, Open With > Text editor of choice.
Why wouldn't you be able to use mouse right click on a mac? Right click the HTML file icon itself.
he needs to view the source of a live web page, not a downloaded html file
Oh. Well, it's more inconvenient on basic browsers like safari, but press CMD + S to save a webpage, select page source as file format, and then open up the .html file with a text editor.