It is the icon share icon. It looks like a box with an arrow comming out of it. You see it at the bottom of the screen in the projects view in iMovie on the iPad; the one that looks like a theater bilboard.
With that icon you can save the movie to the Camera Roll, or even to iTunes using the File Sharing feature.
http://support.apple.com/kb/HT4094
The movie gets encoded if you go the Camera Roll or other export. With the iTunes route it is the project (I'm pretty sure) and you can import and edit it in iMovie on the Mac.
To get to the contents of the camera roll you can use iPhoto, or the Image Capture programs.
IMovie o the Mac stores it's projects in the Movies folder under in your user area. If you dont see it in the Finder select your user name, it should be a subfolder. I believe you have to start at least on projrct for it to be created, though maybe there is a demo project. It has been so lon I am not sure.
I'm not completely sure I understood your question, so if my answere seems a bit scatter shot it is because I was trying to cover all the possibilities. Sorry if I still missed the mark.