Once you're done Installing MantisBT, the web interface is available. However, in order to create/reset user accounts, you need to have email configured so emails can be sent out. I did the following:
search for "g_smtp_host"
change the following settings (I used one of my existing email accounts):
$g_phpMailer_method = PHPMAILER_METHOD_SMTP;
$g_smtp_host = 'smtp.example.com';
$g_smtp_username = 'firstname.lastname@example.org';
$g_smtp_password = 'THIS_IS_CLEAR_TEXT';
$g_smtp_connection_mode = 'ssl';
$g_smtp_port = 465;
At this point, I was able to send email to my account and get logged in with an account other than the default administrator account.
While MantisBT did well at the install, the configuration interface leaves a few things to be desired. First, there are settings beyond getting the system to use the web server or database that cannot be configured in the web gui. Secondly, while all of the possible settings are well documented, grep is your friend; there is no clear indication where they are located.
Total score for ease of administrative configuration: 3/10
Major failing: unable to edit full config from the web interface.