I suppose that your problem is that you don't have a scroll-wheel since you're using a touchpad...
You need to go to "File" > "User preferences" > go to the "Input" tab and on the left you can check the "emulate 3 button mouse" checkbox.
Once you've done that, each time you press Alt+LBM, blender will consider it as being a scroll-wheel clic. ;)
Don't forget to save your Blender User Preferences at the bottom-left of the window.
