ISO is adjusted through a menu or sep. button. Some newer cameras have auto iso. Check your manual
(sorry, been dyin' to use that!
), it's usually pretty straight forward.
In manual mode, you have control of aperature (that's the f/ thingie) and shutter speed, and I doubt any auto-iso works in manual mode. The iso issue is very camera specific, and I'm sorry, but I'm too lazy to look up the XTi manual!