login at BIOS - if you mean SMS menu - just press 1 as the screen starts typing at you.
You do not need to mount the DVD in advance - if you are already running, and you want to install more just use:
# smitty install
and, probably, press ESC-4 (Escape key followed - quickly - by the 4 key, to emulate pressing F4 key) to select the source (choose /dev/cd0).
OOPS:
This means boot the system using the DVD! I wrote a tutorial in pictures
HOWTO: Install AIX - maybe this will help.
Once the base in installed
This all assumes that there is a virtual /dev/cd0 with either a physical /dev/cdX mapped to it, or an iso image loaded from a VIO server.
HOWEVER, if by BIOS you mean ASMI (Advanced System Management Interface) - the user name is admin. If you do not know the password you will need to either reset the system by removing the battery in the service processor (not something I would want to do personally) or open a call with IBM and get a username and password that can be used to reset the regular password.
I have never done the later myself, but a friend of mine needed to do this about a month ago - and that is how he got it resolved. Takes just a few minutes once you get the callback from IBM.
Hope this helps