LOTS of possibilities here... Where do I start,
your SURE the guitar output works?
Are you plugging into the Mic/Instrument inputs (1 or 2)
Are you sure the instrument cable between guitar and interface is good?
When you plug in, and turn up the gain (let's assume your in input 1) for channel 1, that if you CRANK it all the way and strum the instrument LOUDLY that there is a red clip light comes on for channel #1? This would indicate that the sound is getting into the device, now you need to find out why you have no sound...
Have you tried the headphone out? Is there any sound there? If were talking sound from the computer have you set up the Audio Midi ?