1. phpbb is at version 2.0.21 atm, so the archive script shall work with a vanilla version (no mods). This is a common practice to not support exotic setups, so I'd suggest you to try the archive script on a clean phpbb 2.0.21 installation.
The warning you see is probably related to your mod as the "album_mod/album_constants" part of it is suggesting. I think this is more an apache problem (my default Apache in Slackware 10.2 does not display warnings in the browser, but writes them to the log file instead). Playing a bit with the config files of Php and Apache might settle that.
2) If you actually have the files /home/httpdocs/archive/adds/add1.htm and /home/httpdocs/archive/adds/add2.htm (please check it one more time) and if you run an Unix flavor (some Linux or BSD for example), this might be a file permission problem. I could assume that you are seeing these files (if they exist) in a shell/ftp client beinng logged in with your user name, but the Apache daemon (which often runs as 'nobody') doesn't have read access to these files.