hmm... wish I could offer some help but I only use Amplitube as an effect after I've finished recording.
I found that the latency is too much to be able to use it live. ie, my latency is about 23.x at best, which is like playing with an echo on everything.
I usually end up recording straight line in most of the time and then checking it back later to see how it sounded.
As far as actually getting a sound to begin with I'd have to know your software and card. I know on my Mac I have to click the 'sound thru' feature in the audio control panel in Cubase (which only works on OS8.6) and use 'Direct Monitoring' with 'Global Disable' selected. (which probably doesn't help if ya aren't using Cubase.)

I know there's another app called Amplitube Live that I assumed tackled all these problems... never checked it out though. I just kinda live with it the way it is.