Connecting a traditional mouse with a scroll wheel to Mac OS X can be a frustrating experience if you use the default scroll direction (natural) of the Mac OS X trackpad. The scroll wheel also follows this natural direction, which is backwards from what most people expect. If you’re using only the mouse and don’t expect to use the trackpad, then simply reverse the direction under System Preferences > Mouse, and uncheck Scroll Direction: Natural.
However, if you wish to use both, you’ll need an additional tool. Download the free Scroll Reverse software. Unzip the contents into the Applications folder.
Now, you’ll have a double arrow in the system toolbar. Right click and make sure “Reverse Scrolling” is selected.
Next, click Preferences and uncheck “Reverse Trackpad.” You might also want to check “Start at Login.”