You can read this article, but I'm pretty sure since the default terminal in Mac OS X has a built-in scrollbar, the mousewheel commands automatically go to it. You could definitely use gVim as suggested in the previous answer. I find that I don't generally want to use the mouse in Vim though as it takes my hands off the keyboard.
I just use 50j to go down and 50k to go up. Not exactly scrolling, but it works pretty well.
I would recommend using iTerm - it has so many advantages over Terminal eg Mouse support, 256 colors, sensible copy and paste (auto-copy, word/url selection with double click, middle click paste)...
This is an old question, but a top hit on google, so I feel compelled to provide an updated answer.
Running OSX El Capitan 10.11, vim mouse and trackpad scrolling just worked(TM) for me in Terminal.app by default. However occasionally the mouse/trackpad input stopped manipulating the vim buffer, and started scrolling the terminal buffer. The answer was Command+R or Menu View --> Allow Mouse Reporting. Turning that on allowed the mouse/trackpad scroll operations to move the cursor in vim.
Make sure the terminal is xterm & not ansi in Terminal Menu > Preferences > Profiles > Advanced. I accidentally broke scrolling by changing the term type in a naive effort to get coloring to work over ssh.