It depends on what kind of GPU your Mac is running. If it is Radeon, then you can just download your choice of bitcoin mining software, like CGMiner or BFGminer for example, and begin mining. If it does not run the Radeon chipset, then you probably won’t have much hashing power to mine profitably.
Also, I would NOT recommend mining on a laptop as they do not dissipate heat well and you have far more potential to burn out your laptop and lose that investment than you do by making any money mining. The next best choice would be to find a used FPGA rig and attach it to your Mac through USB and it will do all the heavy lifting.
However, spend your money wisely as the difficulty and hash rate of the network will continue to rise as more ASIC’s come online.Here’s a very good list of expected hash rates of current equipment on the market: https://en.bitcoin.it/wiki/Mining_hardware_comparison
Here is a good calculator for predicting the profitability of your mining equipment:http://www.bitcoinx.com/profit/