I have a strange error here that I do not understand

When I changed the line of code to read:

var mainText:TLFTextField = TLFTextField(e.currentTarget);

That cured the problem.

Based on the error, e.target is a SimpleButton object, and you cannot force that into becoming a TLFTextField.  If there is a textfield inside a SimpleButton object, you are likely up a creek without a paddle in terms of being able to target that textfield.  SimpleButton objects have an extremely limited number of properties and cannot have new ones dynamically assigned like MovieClip symbols can (this assumes CS5 hasn't changed this aspect).  So if you have a button that has a textfield in it and you want to be able to target the textfield, you either need to build a class that extends the SimpleButton to have a textfield property, or use a MovieClip instead.