Do you have that in your 'users' file ? (by default /var/lib/octopussy/conf/users.xml)
<?xml version='1.0' encoding='UTF-8'?>
<Octopussy_users>
<user language="EN"
login="admin"
menu_mode="ICONS_AND_TEXT"
password="$1$OP$DS01VEfxN0orlyWumUTfE1"
restrictions=""
role="admin"
status="Enabled"
theme="DEFAULT"
type="local" />
</Octopussy_users>
If yes, the login/pwd is admin/admin...
Check also that you have the good owner/rights for this file:
-rw-r--r-- 1 octopussy octopussy 493 2007-02-19 10:05 users.xml
and that your apache daemon is launched with octopussy user
ps -edf | grep apache
Check your /var/log/apache2/octopussy-error.log file to check where the problem is.
Take a look at the XML::Simple documentation here.