What system did you install it on? Try logging in with your unix usermame and password. ie.; if your servers dns name is localhost.localdomain for example, try logging in as
postmaster@localhost.localdomain or postmaster@localhost or if a true domain
postmaster@somedomain.com. When you setup a unix user the system creates a postmaster account by default.
If these do not work try postmaster@ipaddress
Hope this helps