Tor's project page had this to say:
You'll need to set up our package repository before you can fetch Tor. First, you need to figure out the name of your distribution. If you're using Ubuntu 9.10 or 10.04, it's "karmic", while 9.04 is "jaunty", 8.10 is "intrepid", and 8.04 is "hardy". If you're using Debian Etch, it's "etch", and Debian Lenny is "lenny". Then add this line to your
deb http://deb.torproject.org/torproject.org <DISTRIBUTION> main
where you put the codename of your distribution (i.e. etch, lenny, sid, karmic, jaunty, intrepid, hardy or whatever it is) in place of .
Then add the gpg key used to sign the packages by running the following commands at your command prompt:
gpg --keyserver keys.gnupg.net --recv 886DDD89 gpg --export A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89 | sudo apt-key add -
Now refresh your sources and install Tor by running the following commands at your command prompt:
sudo apt-get update sudo apt-get install tor tor-geoipdb
Now Tor is installed and running. Move on to step two of the "Tor on Linux/Unix" instructions.
The DNS name deb.torproject.org is actually a set of independent servers in a DNS round robin configuration. If you for some reason cannot access it you might try to use the name of one of its part instead. Try deb-master.torproject.org, mirror.netcologne.de or tor.mirror.youam.de.