By export i am guessing you have gone on
share>export movie/or export using quicktime??
if this is correct you wont be able to edit them as these are the video files ready to be published...
when editing videos in iMovie you are working in the project file.
iMovie should save the project file automatically for you in Movies/iMovieprojects/
(this project is only a refference file and doesnt actually save the videos in your project so when you move it you need to make sure you coppy and video or audio added into the project, if that makes sense)
so all of your videos should be saved in Movies/iMovies Events. but have a look to check they are in there.
to edit your movie on another machine you need to copy both of these put them into movies on the new mac and open the project file
hope this helps