The text area merely displays the text and allows for input; if you want it to display data from a text file, then you have to load that text file and send the data to the text area's constructor. Similarly, when you want to save the data, you read the data from the text area and save it to the text file.

I don't have the controlP5 library, but I imagine that something like this would work:

1. String data;
2. String path = "text.txt";
3. //Create a controlP5 object here
   
4. 
   
5. void setup() {
6.       size(400, 400);
7.      
   
8.       //Initialize controlP5 here
9.      
   
10.       data = "";
    
11.      
    
12.       //Load the text file
    
13.       String[] temp = loadStrings(path);
14.       //Iterate through each line
    
15.       for(int i = 0; i < temp.length; i ++) {
16.             //Add a newline character if necessary
    
17.             if(i > 0) data += "\n";
18.             //Add the String data on that line
    
19.             data += temp[i];
    
20.       }
21.      
    
22.       //Initialize the text area here with the preloaded data
    
23. }
24. 
    
25. //etc.
26. 
    
27. void save() {
28.       //Set data to be the text area's value
    
29.      
    
30.       //Convert the data to a String[] and save it
    
31.       saveStrings(path, split(data, "\n");
    
32. }