ntop admin user not created during install

I assume you where using ntop 3.3. Actually the ntop password should have been asked during package installation, unless you were running in non-interactive mode and debconf was not able to ask the password.
However, in the the Debian upload 4.0.3+dfgs1-2 this problem will be fixed. I'll ask for a sync to natty.
Thanks for taking the time to report the issue.

This bug was fixed in the package ntop - 3:4.0.3+dfsg1-2

---------------
ntop (3:4.0.3+dfsg1-2) unstable; urgency=low

  [ Ludovico Cavedon ]
  * Rename maintainer scripts including the package name.
  * Remove misplaced ru.po file.
  * Add Vcs-* headers.
  * Do not call netstat, but read interface list from /proc (Closes: #607637).
  * Use "set -e" in maintainer scripts (see lintian warning
    maintainer-script-without-set-e).
  * Fix bug in check_interfaces() config function.
  * ntop.default: avoid prerm failure if package has never been fully
    configured.
  * Prevent config script failure if password is not asked (Closes: #607580).
  * Cleanup postrm script, thanks to J.M.Roth (Closes: #568626):
    - do not delete user (might be shared)
    - no need to remove /etc/ntop
    - wrap db_purge in a debconf availability check
    - re-indent
  * Add hostname-in-man-description.patch by A. Costa in order to add
    hostname:port in the description section of the man page
    (Closes: #466506).
  * Remove init.cfg from source package, as it is generated by postinst.
  * Handle interface "none", merged patch from Jason Healy (Closes: #520319).
  * Add dependency on net-tools, needed by the init script.
  * Make sure /var/log/ntop is not world readable and set group to adm. Thanks
    to J.M.Roth (Closes: #568641).
  * Set random password admin password if no password have ever been set
    and it is not possible to ask the user (LP: #355127).
  * Suggest geoip-database-contrib and add symlinks to its files.
    Explained in README.Debian.
  * Fix permissions in /var/lib/ntop (LP: #138682). Set sticky bit on
    /var/lib/ntop, so ntop user cannot change init.cfg, but can create
    subdirectories.
  * Disable DNS resolution as it makes ntop unstable.
  * Fix detection of not-active interfaces in ntop.init (LP: #231024).
  * Update po files.

  [ Jordan Metzmeier ]
  * Set password in postinst instead of config (Closes: #609070)
  * Add prompt to reset password if it is already set.
  * Update debconf template thanks to reviews by Christian Perrier and
    Justin B Rye.

  [ Daniel Baumann ]
  * Correct chown calls in ntop.postinst to not fail upon initial installation
    when the respective files are not yet present on the system.
 -- Ludovico Cavedon <email address hidden> Tue, 25 Jan 2011 23:39:56 -0800