Ok, in iPhoto9(mine is v8.1) in an Events window, if you highlight a picture and hit the spacebar, it expands out. The zoom slider is there(bottom right), but greyed out. There is an Edit button(bottom left), click that and you enter the Edit window, and the zoom slider is there.
Instead of using the Spacebar, you can double click your picture and it goes straight to the edit window, with the zoom slider.
If you select the picture you want, from the iPhoto View menu, choose Full Screen, move the mouse to the bottom of the screen, the zoom slider is there.
That's the Edit window, you don't get that if you use the Spacebar option.
Of course if you're using iPhoto 11(it always helps to say which version you have), it could well be different.