For example on the Windows version of Photoshop I can press
Alt -> I -> A -> C in order to get to brightness/contrast.
In my experience, by default, pressing the Alt key on the Mac version does not put the focus on the top menu (though I have been successful in clicking the top with the mouse and using the keyboard from there). I'm specifically referring to the ability to access the top of the menu using one keyboard button press, not the ability to move around when you're already there.
OK, I see. No, the Mac has never used the Option key (its equivalent of Alt) that way. And I don't know of a way to get menu bar focus with a single key; the system shortcut for moving focus to the menu bar described in the article gener7 linked to only works if you enter a key and at least one modifier (Shift, Control, Command, or Option), so that's a minimum of two keys to press, like the Option+Down Arrow combo gener7 set up.