If in doubt go here, for more in deep instructions:
www.i2p2.de
(also for non-i368 and from source installs)

sources.list
/etc/apt/sources.list.d/i2p.list

    deb http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu natty main
    deb-src http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu natty main 

key

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EB2CC88B

install
( some form of java needs to be there, “apt-get installed default-jre”, but i think it comes as a dependency automatically)

apt-get install i2p

start

i2prouter start

it opens in a web-browser, as 127.0.0.1:7657
wait a bit (loot at the left panel, Peers, and wait till a few are listed).
After short i got more than 10 active peers.
*
configure web-browser*
iceweasel→preferences→advanced→network→proxy
http: 127.0.0.1 port:4444
https:127.0.0.1 port:4444

configure i2p
127.0.0.1:7657 → left panel → I2P Services
and have at it.
You might want to set up bandwith in “Network”