Instead of an amp, is it possible to plug in an electric guitar into a Macbook Pro and get the sound from there? Is any special software needed?
Rogue Amoeba's LineIn:
For effects and stuff, GarageBand is probably the best place to start. (Then there's obviously no need to use something like LineIn.) You could also try something like Guitar Rig from Native Instruments.
If you find you're pulling your hairs out because of the latency, you could consider getting a USB or FireWire sound card / audio interface.
A word of caution: it's surprisingy easy to damage both the 3.5 mm plugs and sockets, particularly when you're using a 6.35 mm → 3.5 mm adapter.
Technically speaking, no special software is needed. The audio comes in through a standard cable and all you'll need is the adapter. I can't vouch for the quality though. It is a laptop input after all.