There is an item in the install menu to go to a shell. Selecting this starts up a shell from which you can tinker with the system. Please be aware that before the CD is mounted you will have very few commands at your disposal, and even after the CD is mounted, there are many commands that are unavailable. After packages are installed you will have access to more programs.
If the terminal doesn't look correct when the shell is started, try entering
stty sane
or
setterm -reset
To exit the shell, type Control-d.
If you don't know what you are doing you can easily corrupt the installation process while in the shell. Be careful!