With a working internet connection you can install ethtool with all the usual methods: Ubuntu Software Center, Synaptic Package Manager, apt-get, aptitude etc. I'll try to show how to do this without internet. However, note that for old unsupported releases like yours URLs in /etc/apt/sources.list should point not to http://archive.ubuntu.com/ or its mirrors, but to http://old-releases.ubuntu.com/.
On a network-connected machine, download the deb package for your release and architecture from http://old-releases.ubuntu.com/ubunt...ain/e/ethtool/ (or from http://archive.ubuntu.com/ubuntu/pool/main/e/ethtool/ for currently supported releases).
In your case (Ubuntu 11.04), it's probably either ethtool_2.6.37-1_amd64.deb (64-bit) or ethtool_2.6.37-1_i386.deb (32-bit).
In terminal, you can display the native architecture of your system with
Then install the downloaded package on the target system either by clicking on it in file manager or, in terminal, with
sudo dpkg -i path/to/folder/with/the/package/ethtool*.deb