It sounds like it would be easier, faster, and more reliable to just use the Mac OS serial driver.

But if you really want to bang on the hardware directly, the linked user manual includes the register descriptions necessary to do what you're wanting to do. You'll need to compute the proper clock multipliers for the 2400 baud rate, and the rest of the initialization should be similar to what I've pasted.

The receive code will be similar to the transmit code I pasted, you'll just be polling on the receive status register until a byte is ready, then read it.