A wild guess is that you installed a kernel with devfs, or one that expects devfs to be present, when your system wasn't set up for it. I hope that isn't what happened, and that you can back out of it. If it was, your /dev directory might have been overwritten, which equals headaches from hell to rebuild it. You should be able to tell by looking at your /dev directory.
If so, it's probably easier just to backup your important stuff, then reinstall, letting your installer add devfs information for you. I did this myself about two weeks ago, and just for a challenge, I went ahead and rewrote my /dev, but that's mostly because I'm a little nuts. It's not for a newbie or the faint of heart.