i dont understand what the exact problem is....

you have to be root to install additional software. you can change to root in your shell if you type the command "su" and the password. (not sure what the password for knoppix root is, though. as i remember it was empty).

after this you are root and you can do apt-get update and apt-get install ...

but maybe you wanna try kpackage within KDE. set it up to use apt-get and you can simply klick and install the software you like.

or another fine tool for the shell aptitude. install this one first (apt-get install aptitude) and run it under root. you will have a list with avaiable software and you can search via vi shortcuts.

have fun.