Yeah, the answer seems to be given already. Two further thoughts, fyi:
1. When you create a symbol and then drag it onto the stage, that creates an instance of the symbol in the movie. In order to manipulate that instance with actionscript, you have to give it an instance name (name an instance by selecting it and typing in an instance name in the Properties panel).
It's also helpful to trace your instance names just to be sure they are defined:
If you get 'undefined' traced to the output window, then you know the instance isn't getting defined somewhere along the way (in which case, obviously it needs to be defined).
2. When tabbing, sometimes the cursor will be blinking in a field, but it doesn't seem to respond to some or all of your keyboard typing. In such cases, try clicking your mouse in the field and trying to type something again. Other similar and odd things occasionally happen in such situations, so be aware that your code may be working right after all.