video collage

When on the edit page, how do you import video when you select the "video collage" effect? To me, a video collage implies that you can have several videos (or stills) showing at the same time. I can control the number of boxes that appear, but I can't figure out how to import multiple videos (or stills) to fill in the boxes. When I go to the media page to import a clip, the boxes disappear.

The effect isn't filled with clips, rather applied to a stack of clips, then each clip is assigned a postion as tile, or background.

Good Luck