Perhaps another way to skin the cat would be easier? Your current code is not worth the trouble.
// .Net 2.0
string[] psts = Directory.GetFiles(".", "*.pst", SearchOption.AllDirectories);
// .Net 4.0+
var psts = Directory.EnumerateFiles(".", "*.pst", SearchOption.AllDirectories);
Used like so:
MessageBox.Show(String.Join(", ", psts));