On your Mac, click the Apple logo up at the top left corner of the screen, then 'About This Mac,' the first option in the Apple menu. This will not only show you how much memory you've got in your computer right now, but what type of memory it will take (whether DDR2 or DDR3) and what speed the memory runs at (whether it's 800mHz, 1333mHz, or something else altogether.)
You will need to match the type of memory your Mac takes -- if it uses DDR2 memory, you will not be able to use DDR3 memory, and vice versa. Your best bet is to match the speed, too -- faster RAM will slow itself down to work with your machine, but will be more expensive than necessary. Slower RAM will slow everything down, so using it is not advisable. So, ultimately, match the RAM that your Mac reports it is using.
