have you tried to log in through the login box in the main page?
If I remember correctly the lastest version od PostNuke has just 1 login box, while the older versions had a different login for the admin.
If you are using the "Rogue" version, then you need to login via the main page. then look at the "Main Menu" section and you should see a link named "Administration". If that does not work, try reinstalling PostNuke.