The iframe is inside the game.
I hadn't found that demo till yesterday, but that's the one that taught me that I could call it by it's id. It's a very good demo and gave me a better understanding about what's going on and what am I doing (I'm not a javascript programmer), but I can't still get that string and store it in a variable.
I don't think the problem is about storing. I can store any defined string using JavaScript, something like :
runtime.globalVars.New = "Hello."
Works fine. But, for some reason, I can't get anything inside the iframe. I've tried to use getElementsByClassName(), getElementById and now this contentWindow, but nothing is working. It's really weird to me as I can select with mouse and copy the text inside <span class="letters"> while on preview, so I think it's working fine as HTML code... but I can't acces to it using JavaScript somehow.
Thank you very much for your time. I'll be thinking about this again today as I need to solve it for a very important project.