One standout new feature in the updated user interface is the ability to hover your cursor over a thumbnail of one of the clips in the media manager window, and then a larger, easier-to-see version of that clip enlarges in front of it, .
the scrub tool is just a horizontal fader that should be at the bottom of the Media Manger view if set to thumbnail view. Adjust the fader to adjust the size of the thumbnail being displayed. The expanding thumbnails happens when you move the cursor over a thumb nail. The thubnail will appear to pop out and get larger.
Ah, ok -- so that's a misunderstanding, then. I see the scrub tool at the bottom, but had assumed (hoped) that the enlarged thumbnails would have a scrub tool to seek inside them.
If you right click on an event in the Media Manager, then pick Choose Event, a little window will open with a scrubber beneath the image and you can go through it very quickly---I think that this is what Charlie White was referring too.