Unfortunatelly the Debian Squeeze package repositories do not include the node.js package, so the best way is to install from soures. I created a simple script that will let you install node.js with this one-liner :
sudo bash -c "$(curl -fsSL https://raw.github.com/kkoston/deb6-node-inst/master/go.sh)"
Just copy, paste and execute on your server. I am also hearing that recently released Debain Wheezy repos do not contain node.js package, so that should also work there. If you are experiencing problems with this script, make sure that you have git and curl installed on your system.