I would advise using the real name mod instead. I wrote that tutorial for an earlier version of phpBB. The real name mod v.1.1.0 is better.
Reasons why it's better:
- It is written for phpBB2.0.4 (the latest version) so doesn't need you to read through lots of posts to make sure it works for 2.0.4.
- It has an admin section so that you can edit the new field from users_admin.
All you need to do is change every instance of realname for the name of the new field. Easy to do before installing mod with a find and replace all in a text editor.
Link to it is in my sig.