Unless the input on your mac
1) has a high impedance to match the one offered by the guitar's pickups
2) has abuilt-in preamp
you'll likely obtain very quiet (and possibily a bit distorted) sounds...
You need an external audio interface, even a cheap one like an M-Audio FastTrack USB: that way at least you'll have a decent signal-to-noise ratio.
I didn't even know it existed! That looks like a very good "cheap toy", it should fill your needs nicely if you're not too demanding and know you will never record anything other than a guitar or a bass.
If you plan on recording with a mic, it just won't do.