How do I read read from and write to a text file?

 public TextAsset asset; // Assign that variable through inspector
 private string assetText;

 void Start() {
   assetText = asset.Text; // The assetText variable contains the text
                           // If you're looking to extract data from that,
                           // I would say go with XML. Otherwise, use the 
                           // System.Regex class
 }

Writing to the text file is quite a bit more difficult. However, here is another method that requires a bit of setup and isn't exactly good practice...make sure to add using UnityEngine at the top. The text file you're reading/writing from needs to go into a special directory called "Resources".

 public string FileName; // This contains the name of the file. Don't add the ".txt"
                         // Assign in inspector
 private TextAsset asset; // Gets assigned through code. Reads the file.
 private StreamWriter writer; // This is the writer that writes to the file

 void AppendString(string appendString) {
   asset = Resources.Load(FileName + ".txt") as TextAsset;
   writer = new StreamWriter("Resources/" + FileName + ".txt"); // Does this work?
   writer.WriteLine(appendString);
 }

In Update() use the AppendString() function to write text.

Yes, System.IO works on Mac