Compositor, AlphaOver and nodes. Help needed

I am now at the compositing level of my learning curve and in the project I work on I have used render passes and render layers.
At the level of rendering separately the objects I don’t seem to have problem, but at the compositing I cannot get the whole scene completed!
Either the background is missing or black, or objects are missing…
I believe this has to do with the alpha that I have activated but I don’ seem to correctly use it… I don’t get it at this point
Any help would be great
I have watched so many tutorials that I feel confused now
The blend file is at http://www.pasteall.org/blend/23132

Without knowing more about your comp tree it’s hard to give some specific comment.

I believe this has to do with the alpha that I have activated but I don’ seem to correctly use it…

Just some random thoughts:
In the Render panel in the Shading area, have you set the Alpha to Transparent? Otherwise the BG won’t be transparent - no alpha.
Your Image output format should support alpha of course (like Exr or Png,…) with the according button selected (RGBA)

Sorry, just noticed you had a blend file attached…

Since it is a cycles scene, there is no shading area in the render panel of course…

With your scene, I don’t see what should be wrong with the alpha’s. Seems to render and comp all 4 bottles…