Why can't see "slider" to scroll images in Split View?

Ok, this is going to be an embarrassingly simple question...

When in the 'Split View' in earlier versions of Aperture (I am now on 3.5.1), I would like to be able to grab the "slider" (I don't know what it's actual name is..) that I would think appears below the images at the bottom of the screen, and then drag it left/right to move through pictures quickly. (just like the slider that appears in a browser window on the right edge that I can use to scroll up, down on a web page).

Right now to scroll through pictures I have to either

1. Roll mouse button over and over again when the cursor is in the bottom view area (with the little pictures), or

2. Click on an image, and then arrow right/left to move

For some reason when i am moving the mouse button over and over the slider will sometimes appear. When I try to "grab" it and use the slider to move and scroll left/right through images, it goes away. Most apps when I scroll a direction, the slider will appear and I can grab it and scroll with it.

One time I was able to grab it and use it to move through pictures. Once I went to another view and back it was gone.
And at other times I can't see the slider at all.

What on earth am I doing wrong??

Yikes

Thanks-