Texture painting alpha material 2.5

Hey. I have a lots of problems to figuring out how to paint an invisible material with blender with a texture painting mode.

There is a box “alpha” in texture painting panel, but it changes to grey if i choose “erase alpha” or “add alpha” blend mode. and if i try to paint something with that “erase or add alpha” brush, it does nothing… I have not idea how to do this but i think that with blender you can do it maybe.

I cant ever find anything usefull feature in blender that i need without watching a tutorial or asking in some forum about it… It’s driving me crazy… Too much thinks that you must remember and every time you must go about in 3 different places for making settings before something works.

I am not seeing “alpha” in texture painting panel. Shoot me a screen shot will you?

I mean this: