In heidisql (or phpmyadmin)
Go to the table you want. Now when the page loads, you should find this "add column" button or something similar.
Just enter your values in.
Now, in database.cs
I'll use public Account Register(string uuid, string password, bool isGuest, XmlData data) as an example.
To add a new column for this,
just add
Code:
cmd.Parameters.AddWithValue("@column", valuetype);
under
Code:
cmd.Parameters.AddWithValue("@empty", "");
If you read through the file, you should get the flow as to how it works
(sorry for a poor tutorial. I don't have phpmyadmin or heidisql to show you how it looks like)