Depending on how you have your mouse configured try pressing the roller wheel it should turn to a pan icon on the screen. hold and move cursur to the center of the screen. Then roll the wheel for zoom.

Considering you said you are using AutoCAD LT you will not have much control on how you can custmize. LT is a stripped down version of AutCAD. Thats why it was much cheaper than the full version. Hope this helps.

Ozzy