Flash Player/Browser Problem

Over the past couple of months I´ve been creating a website and had some problems along the way.
After lots of tests regarding different issues, I realized the problem in all cases was the same:

I can not get to any MC in _level0 (main swf) from an external swf´s.
Which you know, should be easy, something like _root.MenuMC._visible = false; should work
and it does, sort of.
It works when I test my movie within flash (ctrl+enter or F12), but it does not work
when I test in on the browser.

I´ve tried many different ways to do it, like:
and so on…

Nothing works and I have no idea what´s causing that problem or how to fix it.

Does someone know about this?

Thanks a lot, Marcelo.