It's right-click or control-click on Mac.
But @jims24, as far as I know the addition of 'sort' to this menu was only done in Opera 32, so you'll have to wait until that version is released as stable (should not take that long) or use an unstable version from the developer stream in the mean time (see http://blogs.opera.com/desktop for developer stream builds). The post you saw in the Windows forum was probably also referring to someone using the developer stream (the menus are the same on Windows and Mac).