If the data apply at the whole addin, regardless of the user logged in, I suggest to write your XML file in the folder defined by Environment.SpecialFolder.CommonApplicationData
string commonPath = Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData));
string myAddinDataPath = Path.Combine(commonPath, "MyAddinName");
if(!Directory.Exists(myAddinDataPath)) Directory.CreateDirectory(myAddinDataPath);
instead, if you have different data based on the current user, write in the the folder defined by Environment.SpecialFolder.ApplicationData