It sounds like a targeting problem. If the text box is in a different movie than the button or the action, then you need to let Flash know where to send the command.

So if you are trying to load text into a field on the main movie from a button in a movie loaded onto level 1 the action would look like: _level0.loadVariablesNum(whatever.txt)

To do it the other way the _level0. would be _level1.

You can also use the instance name of the clip such as instanceName.loadVariablesNum(whatever.txt)