AI Servo in live mode (XSi)

[Egy_lover]

Hi,
Since I bought my 450D (XSi) I've been shooting still objects and Manually focusing all the time! But now I'm starting to learn taking photos of moving things, I'm a beginner and I can't figure out how to shoot moving subjects (always out of focus), I'm not talking about fast objects, just walking children, people, in a medium or even slow speed. It is very hard to keep manually focusing all the time.

Back when I used a P&S (SX 200) I used face detection, which draws a square around faces and with AI servo this square moved on the LCD as the subject moves. Why isn't this happening on my 450D? (I think I sound naive now!)...

The AI servo is working when I use the Viewfinder, it keeps changing focus as the subject moves, but is it possible in the live view (Live mode) ?

I've read the manual but I didn't get the answer.
Can you please help?