IIRC it's a similar process, haven't checked it out though. It depends how you were doing it in the older version. For me here's what I do:
Place the playhead at the part of the clip I want to create a still from. Right-click (or control-click) at that point on the clip. In imovie 7, the option came up to add a still frame, but in imovie 9 I think it's called "Add Freeze Frame"?
It then adds the still. You can then also right-click the still and get it revealed in Finder so you have the still to add in jpeg to iPhoto etc.
Dunno if that's how you've tried, I haven't used iMovie in a good while.
Also note, with quicktime, you can just pause the movie, then click and drag the still image to your desktop. That way you get the image and don't have to do a screenshot.