Attach movie preloader

Hi!

I have a little problem, How do I make a preloader if I have only attached movieclips from the library, I know how to make a preloader but not for attached movieclips. The movieclips are attached from level 0 to 12, and I want one preloader to preload all levels at once, how do I do that? , Tried with a for loop to loop through all levels but it didn´t work.

Thanks
Rebus