You need to use the loadVariables command for this.
On the frame of your movie from which you want to call the textfile put this line into your actions panel...
loadVariables("textfile.txt",0);
The only other things you need to have set up are the format of the textfile and the dynamic box on the stage...
Make sure that the first thing in your textfile is the variable you want to use in your movie, e.g. ...
myVariable="the text for my movie";
Give your dynamic textbox the variable name myVariable and it will display the contents of your textfile.