Even though Ubuntu Karmic Koala comes with the couchdb executable already installed, you still have to do "sudo aptitude install" to install the full package to use it for web development. You also have to create a special dir as a workaround for a bug. Here are the full steps:
sudo aptitude install couchdb
sudo su
mkdir /var/log/couchdb/0.10.0
chown couchdb:couchdb /var/log/couchdb/0.10.0
exit
sudo -i -u couchdb couchdb -b
The last line is to run couchdb as a special user role called, "couchdb", which has limited privileges. The second part of the line, "couchdb -b", is the actual command being executed by the user, "couchdb", to run CouchDB in a background process. You kill the CouchDB process by using "-d" instead of "-b":
sudo -i -u couchdb couchdb -d
After all that, open your brower to: http://127.0.0.1:5984/
You should see:
{"couchdb":"Welcome","version":"0.10.0"}
Full info. at: https://bugs.launchpad.net/ubuntu/+source/couchdb/+bug/453049
Sidenote: Looking to see how amazing CouchDB really is? Take a look at some examples: http://www.jasondavies.com/blog/tags/couchdb/