On my Mac OS X system (10.9.1), the SDL_MOUSEWHEEL event reports negative X values when my trackpad scrolls to the right, and positive X values when my trackpad scrolls to the left. This is backwards from what I'd expect, and I don't think it matches the Windows wheel events.

The vertical scroll values are what I'd expect though, and are consistent what gets reported on Windows (positive Y for scrolling up, negative Y for scrolling down.)

This is with "scroll direction: natural" disabled in the OS X trackpad settings (i.e. my scroll direction in non-SDL OS X programs matches what happens in Windows and Linux.)

I also tested with the horizontal scroll on a real mouse (Logitech G500 without custom drivers), and the horizontal scroll values in SDL are still flipped.

I "solved" the issue for myself by changing this line in the Cocoa_HandleMouseWheel function:

    float x = [event deltaX];

to this:

    float x = -[event deltaX];

I believe it should work fine with that change - I found something similar in another codebase while looking online for my issue - but I haven't tested on anything below Mac OS 10.8.