First, get an app called MPEG Streamclip, which is free. (google MPEG Streamclip from Squared 5)
Open MPEG Streamclip.
In iMovie, select the clip you need. Then, right-click/Reveal in Finder.
Drag this clip into MPEG Streamclip
In MPEG Streamclip, move the playhead to the frame you want.
In MPEG Streamclip, click FILE/EXPORT FRAME.
Choose JPEG, TIFF, or PNG and give it a name.
You can then drag this photo into iPhoto, Aperture, or anywhere in the Finder.
You should not be looking for a stills folder. You should select the video clip in either the Project or the Event. Then right-click/Reveal in Finder. The video clip should be highlighted in blue in the Finder. Drag this video clip to MPEG Streamclip. Move the cursor to the frame you want, and FILE/EXPORT FRAME.