The buttons are populated by the by the IR ID (the grayed out box next to the Button ID). There is no way to directly enter your own IR ID you can however copy a button from the Alpha template then paste just the IR ID into your own button.

Create your own button then copy (CTRL c) say the GUIDE button from the template then right click your button and click paste special. Then uncheck all the boxes except for the IR ID box, then click OK. You will now notice that the IR ID will have changed from being blank and now has the same IR ID as the guide button.

Hope this is some help to you. I've been trying to find a list of what each code is, but have been unsuccessful.