What's Taking Up My Memory And Why Can't I Find The Files?

I am editing a movie in iMovie. I have edited a few documentaries already, but it appears even if I delete files from iMovie they not only don't go away, there is no way to find them to delete them. I cannot tell what is taking up all my space in my documents either. Since I can't seem to be able to open iMovie from an external drive (Lacie), that option isn't there.

I have already ruined one project trying to delete and move files around to free up storage.

Without the access to what is taking up my storage, and the external drive option, this iMac isn't much good for editing videos. If someone can't offer me some solution that works, I'll have to get a new computer just so I can have access to the memory it's supposed to have.