Help! Dynamic Text Box Masking

Hi all. So i have this weird problem. My main movie loads an external SWF. In this SWF is a MC which has a Dynamic Text box in it. I wanted it to animate in as the SWF is loaded on to the main stage. It starts off screen and slides on to the stage. However when I play the main movie and load the external SWF I can see the text that is supposed to be off screen now on screen because the main stage is larger. So, i tried to add a mask, but when i do that the text box just doesn’t want to show up. I am guessing this is because the text is dynamic in nature.

Is there another method to mask it? Or make it ‘slide out’ from behind something?

Cheers

-Zey