Are th foiles you're modifying in the correct directory and have the correct names? I don't use Ubuntu so I don't know if the following is correct:
Installing An Ubuntu Hardy 8.04 LTS DNS Server With BIND - Page 4 | HowtoForge - Linux Howtos and Tutorials but it seems to imply the files are in a different location than the one you're modifying.
If you're having problems with it then you might like to install
webmin to manage your server, it also has a DNS configuration page which may help.