First off, I could never recommend buying a Monster cable. They're outrageously overpriced and no better than moderately priced cables.
In particular, if you're planning on using the line/mic input on your Mac, it would be a tremendous waste of money to spend anything other than nominal amounts on a cable.
You really should use some kind of preamp between the guitar and computer. If you have a passive pickup, the signal is likely to be too low and if you have an active pickup (like most acoustic/electric guitars), you're likely to overdrive the input circuitry in the computer without some kind of preamp.
I would highly recommend looking at USB/Firewire interfaces for getting instruments/vocals into your computer. Also, if you're trying to record acoustic guitar, the sound will be a lot better with a microphone. You might want to take a look at something like the Blue Snowflake (nice USB condenser mic). It can be found for under $50.