Logic Studio 2 (Mainstage) usage

There are a few possibilities. You can get hardware from IK Multimedia (Amplitube) or Native Instruments (Guitar Rig); that would allow you plug in your guitar and act as an audio interface. Or you can get a simple MIDI pedalboard (for example this one from Roland) to switch patches and get a dedicated audio interface to get your guitar sound to the computer.

On the other hand I decided against going computer for the guitar. I play keyboards as well, and thought of routing my guitar through the computer too but my experience is that there is too much trouble with it. The aforementioned hardware is USB only, so you are sentenced to a maximum of 5 meters of cable. A real audio interface can be gotten in the Firewire flavor as well, and you can put many meters of MIDI cable between the audio/MIDI interface and the MIDI pedalboard.

As I think 5 meter is too small, and there were too many problems regarding latency and huge CPU/Memory usage I went back to simply plugging it back into an amp.

Maybe you have better luck, I use Amplitube just as a means to record guitars in the songwriting/arranging stage as it is way easy to change things like chorus or delay speeds and levels of overdrive.

Enjoy.