Changing black background to transparent for lens flare effect

I realise this has been answered in other posts, but I am a complete newbie to After Effects (only 2nd time using it) and couldn't understand the other answers!

So I have After Effects 6, and I have many black solid layers, each with a lens flare effect, each set to Screen, whatever that means. Now, I've toggled the transparency grid, meaning I know that the solids are actually black, not transparent as I can't see the checkerboard pattern through them.

I want to make all the layers transparent so that I can just stick the lens flares over another clip in Premiere Pro. But I can't work out how to do it!

Like I say, I am a complete beginner, so I would be really grateful if you could mention under what menu I will find the various options.

Many Thanks

Sally :)

It seems Knoll Unmult only works for up to CS5 - do you know another extension that works for CS6 as a one click solution would be brilliant!
Thanks
Sally

It worked!
You, sir, are a genius and lifesaver :)
Very many thanks
Sally :)