You can't do this from a console within X?
No need to change the default init. That's the beauty of SysV. All you have to do is grab a VC, and you don't have to do that, but you'll probably think you crashed the box if you don't then switch from init 5 to init 3.
So grab a VC, just as HigherGround describes. And, as root, issue init 3. This will switch you to run level 3. Install the binaries, then issue init 5 to go back to graphical mode. You'll need to switch back to tty7 to see your kdm login screen, and you're done.