Add the following to your /etc/apt/sources.list
deb http://ftp.us.debian.org/debian squeeze main
deb-src http://ftp.us.debian.org/debian squeeze main
deb http://ftp.us.debian.org/debian squeeze-updates main
deb-src http://ftp.us.debian.org/debian squeeze-updates main
deb http://security.debian.org/ squeeze/updates main
deb-src http://security.debian.org/ squeeze/updates main
or just use a complete list is here: click here and scroll down to #5 - how to update Debian.
Command: apt-get update && apt-get install acroread mozilla-acroread
Enjoy!
