a) Download latest rootkithunter from http://www.chkrootkit.org/download/ (latest version is chkrootkit-0.49 but its has bugs)
mv chkrootkit.tar.gz /usr/local/
cd /usr/local/
tar xvfz chkrootkit.tar.gz
ln -s chkrootkit-0.43/ chkrootkit (replace 0.43 with the right version number)
cd chkrootkit/
make sense
You will now find the chkrootkit program under /usr/local/chkrootkit. Run it by typing
cd /usr/local/chkrootkit/ && ./chkrootkit