How to make a colour transparent in either Motion 5 OR Final Cut Express?

Hi, I have two video files, one with a series of circles on a black background and the other with a series of circles on a white background. The white file is the background while the black files is the foreground. All I want to do is make the black of the foreground file so it looks like the foreground circles are over the background circles. (I do not want the foreground circles to be transparent at all, I just want the black gone) I have been able to make literally every colour circle transparent so it only shows the background, but the black is still there, so it's essentially the exact reverse of what I want. I hope this makes sense, and I REALLY hope someone can help. It doesn't matter if it's in Motion 5 or Final Cut Express HD, I just want to know how to make that black transparent while leaving everything else as it is. And please, explain the menu options and locations like I have never used a computer before. The rare few times I found any help ding a project, the poster always posts names, tabs and drop down menus that are not at all in the same places as they say it is. Thank you to anyone that can help!