Normally, mask setting in Flash IDE don't work for dynamic TextField and TextInput components. With default settings, you'll not be able to see the text through the mask. People using masked layers from Flash IDE often fall into this problem. One stupid solution to this problem is not using mask at all. However, if you want to implement something like custom scrolling, sliding etc. then most of the time you'll need to use mask.
Masking Test & Input Fields in Flash
A common problem in Flash involving text and masking. Check it out!