I have tried to implement the shoutbox on my phpbb2 version 2.0.23 and I did everything to the letter.
After I completed all stages I did not see anything appearing on my main board, so I went on to check the Administrator Panel, guessing there would be an added option there.
When I went to the Administration Panel, much to my surprise the main body was ok, but the left sided menu is nowhere. I get an empty frame there, consequently rendering me unable to do ANYTHING administrator wise, to my board, from config, to users, to groups etc...
I reverted to my old files which I backed up before the installation attempt, and even removed the 2 new tables from the SQL plus the values added in the phpbb_config but the Admin left sided frame is still empty. I refreshed my page numerous times, I even cleared all cookies and so on, but still the left frame on the admin panel is empty.
This could be caused by many things, but it sounds like an error is occurring in a script causing the page to die but it's not showing you any errors. Does the page show you any errors, or just a blank page?