This is not what you asked. What does:

(1) Read available text files in the current directory
(2) Choose one file (e.g., mTextFile.txt) and open it
(3) Save the mTextFile.txt to a text box (editing file in text box).
(4) Save the changes to either the same text file or to a new textfile

have do with Active Document? If nothing, please state a bit more precisely what you actually do want to do. For example #4 is rather vague...that "or" make quite a bit of difference.

Plus you do not state if the textbox exists already, or are you making a new textbox. I really do not know what you mean by "editing file in text box". You do not edit any files in text boxes, you can change a string in a textbox.